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{"kind": "coding_drills", "role_id": "role_1ZSkiZiwtQg", "source": "leetcode_graphql", "generated_at": "2026-05-21T01:56:54.222427+00:00", "difficulty_mix": {"Easy": 3, "Medium": 5, "Hard": 2}, "topics_filter": [], "questions": [{"index": 0, "category": "coding_array", "question": "[Easy] LC #14 \u00b7 Longest Common Prefix \u2014 solve on leetcode.com", "why": "Topic tags: array, string, trie. AC rate 47.6%. Open the editor at https://leetcode.com/problems/longest-common-prefix/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "longest-common-prefix", "frontend_id": "14", "difficulty": "Easy", "ac_rate": 47.5629941623571, "topics": ["array", "string", "trie"], "url": "https://leetcode.com/problems/longest-common-prefix/", "problem_html": "<p>Write a function to find the longest common prefix string amongst an array of strings.</p>\n\n<p>If there is no common prefix, return an empty string <code>""</code>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> strs = ["flower","flow","flight"]\n<strong>Output:</strong> "fl"\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> strs = ["dog","racecar","car"]\n<strong>Output:</strong> ""\n<strong>Explanation:</strong> There is no common prefix among the input strings.\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= strs.length <= 200</code></li>\n\t<li><code>0 <= strs[i].length <= 200</code></li>\n\t<li><code>strs[i]</code> consists of only lowercase English letters if it is non-empty.</li>\n</ul>\n", "hints": []}}, {"index": 1, "category": "coding_array", "question": "[Easy] LC #26 \u00b7 Remove Duplicates from Sorted Array \u2014 solve on leetcode.com", "why": "Topic tags: array, two-pointers. AC rate 62.8%. Open the editor at https://leetcode.com/problems/remove-duplicates-from-sorted-array/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "remove-duplicates-from-sorted-array", "frontend_id": "26", "difficulty": "Easy", "ac_rate": 62.81073341805187, "topics": ["array", "two-pointers"], "url": "https://leetcode.com/problems/remove-duplicates-from-sorted-array/", "problem_html": "<p>Given an integer array <code>nums</code> sorted in <strong>non-decreasing order</strong>, remove the duplicates <a href=\"https://en.wikipedia.org/wiki/In-place_algorithm\" target=\"_blank\"><strong>in-place</strong></a> such that each unique element appears only <strong>once</strong>. The <strong>relative order</strong> of the elements should be kept the <strong>same</strong>.</p>\n\n<p>Consider the number of <em>unique elements</em> in <code>nums</code> to be <code>k<strong>\u200b\u200b\u200b\u200b\u200b\u200b\u200b</strong></code>\u200b\u200b\u200b\u200b\u200b\u200b\u200b. <meta charset=\"UTF-8\" />After removing duplicates, return the number of unique elements <code>k</code>.</p>\n\n<p><meta charset=\"UTF-8\" />The first <code>k</code> elements of <code>nums</code> should contain the unique numbers in <strong>sorted order</strong>. The remaining elements beyond index <code>k - 1</code> can be ignored.</p>\n\n<p><strong>Custom Judge:</strong></p>\n\n<p>The judge will test your solution with the following code:</p>\n\n<pre>\nint[] nums = [...]; // Input array\nint[] expectedNums = [...]; // The expected answer with correct length\n\nint k = removeDuplicates(nums); // Calls your implementation\n\nassert k == expectedNums.length;\nfor (int i = 0; i < k; i++) {\n assert nums[i] == expectedNums[i];\n}\n</pre>\n\n<p>If all assertions pass, then your solution will be <strong>accepted</strong>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [1,1,2]\n<strong>Output:</strong> 2, nums = [1,2,_]\n<strong>Explanation:</strong> Your function should return k = 2, with the first two elements of nums being 1 and 2 respectively.\nIt does not matter what you leave beyond the returned k (hence they are underscores).\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [0,0,1,1,1,2,2,3,3,4]\n<strong>Output:</strong> 5, nums = [0,1,2,3,4,_,_,_,_,_]\n<strong>Explanation:</strong> Your function should return k = 5, with the first five elements of nums being 0, 1, 2, 3, and 4 respectively.\nIt does not matter what you leave beyond the returned k (hence they are underscores).\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= nums.length <= 3 * 10<sup>4</sup></code></li>\n\t<li><code>-100 <= nums[i] <= 100</code></li>\n\t<li><code>nums</code> is sorted in <strong>non-decreasing</strong> order.</li>\n</ul>\n", "hints": ["In this problem, the key point to focus on is the input array being sorted. As far as duplicate elements are concerned, what is their positioning in the array when the given array is sorted? Look at the image below for the answer. If we know the position of one of the elements, do we also know the positioning of all the duplicate elements?\r\n\r\n<br>\r\n<img src=\"https://assets.leetcode.com/uploads/2019/10/20/hint_rem_dup.png\" width=\"500\"/>", "We need to modify the array in-place and the size of the final array would potentially be smaller than the size of the input array. So, we ought to use a two-pointer approach here. One, that would keep track of the current element in the original array and another one for just the unique elements.", "Essentially, once an element is encountered, you simply need to <b>bypass</b> its duplicates and move on to the next unique element."]}}, {"index": 2, "category": "coding_array", "question": "[Easy] LC #1 \u00b7 Two Sum \u2014 solve on leetcode.com", "why": "Topic tags: array, hash-table. AC rate 57.5%. Open the editor at https://leetcode.com/problems/two-sum/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "two-sum", "frontend_id": "1", "difficulty": "Easy", "ac_rate": 57.49451107379435, "topics": ["array", "hash-table"], "url": "https://leetcode.com/problems/two-sum/", "problem_html": "<p>Given an array of integers <code>nums</code> and an integer <code>target</code>, return <em>indices of the two numbers such that they add up to <code>target</code></em>.</p>\n\n<p>You may assume that each input would have <strong><em>exactly</em> one solution</strong>, and you may not use the <em>same</em> element twice.</p>\n\n<p>You can return the answer in any order.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [2,7,11,15], target = 9\n<strong>Output:</strong> [0,1]\n<strong>Explanation:</strong> Because nums[0] + nums[1] == 9, we return [0, 1].\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [3,2,4], target = 6\n<strong>Output:</strong> [1,2]\n</pre>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> nums = [3,3], target = 6\n<strong>Output:</strong> [0,1]\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>2 <= nums.length <= 10<sup>4</sup></code></li>\n\t<li><code>-10<sup>9</sup> <= nums[i] <= 10<sup>9</sup></code></li>\n\t<li><code>-10<sup>9</sup> <= target <= 10<sup>9</sup></code></li>\n\t<li><strong>Only one valid answer exists.</strong></li>\n</ul>\n\n<p> </p>\n<strong>Follow-up: </strong>Can you come up with an algorithm that is less than <code>O(n<sup>2</sup>)</code><font face=\"monospace\"> </font>time complexity?", "hints": ["A really brute force way would be to search for all possible pairs of numbers but that would be too slow. Again, it's best to try out brute force solutions just for completeness. It is from these brute force solutions that you can come up with optimizations.", "So, if we fix one of the numbers, say <code>x</code>, we have to scan the entire array to find the next number <code>y</code> which is <code>value - x</code> where value is the input parameter. Can we change our array somehow so that this search becomes faster?", "The second train of thought is, without changing the array, can we use additional space somehow? Like maybe a hash map to speed up the search?"]}}, {"index": 3, "category": "coding_two_pointers", "question": "[Medium] LC #5 \u00b7 Longest Palindromic Substring \u2014 solve on leetcode.com", "why": "Topic tags: two-pointers, string, dynamic-programming. AC rate 37.8%. Open the editor at https://leetcode.com/problems/longest-palindromic-substring/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "longest-palindromic-substring", "frontend_id": "5", "difficulty": "Medium", "ac_rate": 37.83336228184537, "topics": ["two-pointers", "string", "dynamic-programming"], "url": "https://leetcode.com/problems/longest-palindromic-substring/", "problem_html": "<p>Given a string <code>s</code>, return <em>the longest</em> <span data-keyword=\"palindromic-string\"><em>palindromic</em></span> <span data-keyword=\"substring-nonempty\"><em>substring</em></span> in <code>s</code>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> s = "babad"\n<strong>Output:</strong> "bab"\n<strong>Explanation:</strong> "aba" is also a valid answer.\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> s = "cbbd"\n<strong>Output:</strong> "bb"\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= s.length <= 1000</code></li>\n\t<li><code>s</code> consist of only digits and English letters.</li>\n</ul>\n", "hints": ["How can we reuse a previously computed palindrome to compute a larger palindrome?", "If \u201caba\u201d is a palindrome, is \u201cxabax\u201d a palindrome? Similarly is \u201cxabay\u201d a palindrome?", "Complexity based hint:</br>\r\nIf we use brute-force and check whether for every start and end position a substring is a palindrome we have O(n^2) start - end pairs and O(n) palindromic checks. Can we reduce the time for palindromic checks to O(1) by reusing some previous computation."]}}, {"index": 4, "category": "coding_math", "question": "[Medium] LC #29 \u00b7 Divide Two Integers \u2014 solve on leetcode.com", "why": "Topic tags: math, bit-manipulation. AC rate 19.8%. Open the editor at https://leetcode.com/problems/divide-two-integers/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "divide-two-integers", "frontend_id": "29", "difficulty": "Medium", "ac_rate": 19.778929080081173, "topics": ["math", "bit-manipulation"], "url": "https://leetcode.com/problems/divide-two-integers/", "problem_html": "<p>Given two integers <code>dividend</code> and <code>divisor</code>, divide two integers <strong>without</strong> using multiplication, division, and mod operator.</p>\n\n<p>The integer division should truncate toward zero, which means losing its fractional part. For example, <code>8.345</code> would be truncated to <code>8</code>, and <code>-2.7335</code> would be truncated to <code>-2</code>.</p>\n\n<p>Return <em>the <strong>quotient</strong> after dividing </em><code>dividend</code><em> by </em><code>divisor</code>.</p>\n\n<p><strong>Note: </strong>Assume we are dealing with an environment that could only store integers within the <strong>32-bit</strong> signed integer range: <code>[−2<sup>31</sup>, 2<sup>31</sup> − 1]</code>. For this problem, if the quotient is <strong>strictly greater than</strong> <code>2<sup>31</sup> - 1</code>, then return <code>2<sup>31</sup> - 1</code>, and if the quotient is <strong>strictly less than</strong> <code>-2<sup>31</sup></code>, then return <code>-2<sup>31</sup></code>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> dividend = 10, divisor = 3\n<strong>Output:</strong> 3\n<strong>Explanation:</strong> 10/3 = 3.33333.. which is truncated to 3.\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> dividend = 7, divisor = -3\n<strong>Output:</strong> -2\n<strong>Explanation:</strong> 7/-3 = -2.33333.. which is truncated to -2.\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>-2<sup>31</sup> <= dividend, divisor <= 2<sup>31</sup> - 1</code></li>\n\t<li><code>divisor != 0</code></li>\n</ul>\n", "hints": []}}, {"index": 5, "category": "coding_hash_table", "question": "[Medium] LC #17 \u00b7 Letter Combinations of a Phone Number \u2014 solve on leetcode.com", "why": "Topic tags: hash-table, string, backtracking. AC rate 66.1%. Open the editor at https://leetcode.com/problems/letter-combinations-of-a-phone-number/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "letter-combinations-of-a-phone-number", "frontend_id": "17", "difficulty": "Medium", "ac_rate": 66.05142014132613, "topics": ["hash-table", "string", "backtracking"], "url": "https://leetcode.com/problems/letter-combinations-of-a-phone-number/", "problem_html": "<p>Given a string containing digits from <code>2-9</code> inclusive, return all possible letter combinations that the number could represent. Return the answer in <strong>any order</strong>.</p>\n\n<p>A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.</p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2022/03/15/1200px-telephone-keypad2svg.png\" style=\"width: 300px; height: 243px;\" />\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> digits = "23"\n<strong>Output:</strong> ["ad","ae","af","bd","be","bf","cd","ce","cf"]\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> digits = "2"\n<strong>Output:</strong> ["a","b","c"]\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= digits.length <= 4</code></li>\n\t<li><code>digits[i]</code> is a digit in the range <code>['2', '9']</code>.</li>\n</ul>\n", "hints": []}}, {"index": 6, "category": "coding_string", "question": "[Medium] LC #38 \u00b7 Count and Say \u2014 solve on leetcode.com", "why": "Topic tags: string. AC rate 62.9%. Open the editor at https://leetcode.com/problems/count-and-say/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "count-and-say", "frontend_id": "38", "difficulty": "Medium", "ac_rate": 62.880196768473894, "topics": ["string"], "url": "https://leetcode.com/problems/count-and-say/", "problem_html": "<p>The <strong>count-and-say</strong> sequence is a sequence of digit strings defined by the recursive formula:</p>\n\n<ul>\n\t<li><code>countAndSay(1) = "1"</code></li>\n\t<li><code>countAndSay(n)</code> is the run-length encoding of <code>countAndSay(n - 1)</code>.</li>\n</ul>\n\n<p><a href=\"http://en.wikipedia.org/wiki/Run-length_encoding\" target=\"_blank\">Run-length encoding</a> (RLE) is a string compression method that works by replacing consecutive identical characters (repeated 2 or more times) with the concatenation of the character and the number marking the count of the characters (length of the run). For example, to compress the string <code>"3322251"</code> we replace <code>"33"</code> with <code>"23"</code>, replace <code>"222"</code> with <code>"32"</code>, replace <code>"5"</code> with <code>"15"</code> and replace <code>"1"</code> with <code>"11"</code>. Thus the compressed string becomes <code>"23321511"</code>.</p>\n\n<p>Given a positive integer <code>n</code>, return <em>the </em><code>n<sup>th</sup></code><em> element of the <strong>count-and-say</strong> sequence</em>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">n = 4</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">"1211"</span></p>\n\n<p><strong>Explanation:</strong></p>\n\n<pre>\ncountAndSay(1) = "1"\ncountAndSay(2) = RLE of "1" = "11"\ncountAndSay(3) = RLE of "11" = "21"\ncountAndSay(4) = RLE of "21" = "1211"\n</pre>\n</div>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<div class=\"example-block\">\n<p><strong>Input:</strong> <span class=\"example-io\">n = 1</span></p>\n\n<p><strong>Output:</strong> <span class=\"example-io\">"1"</span></p>\n\n<p><strong>Explanation:</strong></p>\n\n<p>This is the base case.</p>\n</div>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= n <= 30</code></li>\n</ul>\n\n<p> </p>\n<strong>Follow up:</strong> Could you solve it iteratively?", "hints": ["Create a helper function that maps an integer to pairs of its digits and their frequencies. For example, if you call this function with \"223314444411\", then it maps it to an array of pairs [[2,2], [3,2], [1,1], [4,5], [1, 2]].", "Create another helper function that takes the array of pairs and creates a new integer. For example, if you call this function with [[2,2], [3,2], [1,1], [4,5], [1, 2]], it should create \"22\"+\"23\"+\"11\"+\"54\"+\"21\" = \"2223115421\".", "Now, with the two helper functions, you can start with \"1\" and call the two functions alternatively n-1 times. The answer is the last integer you will obtain."]}}, {"index": 7, "category": "coding_array", "question": "[Medium] LC #11 \u00b7 Container With Most Water \u2014 solve on leetcode.com", "why": "Topic tags: array, two-pointers, greedy. AC rate 60.0%. Open the editor at https://leetcode.com/problems/container-with-most-water/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "container-with-most-water", "frontend_id": "11", "difficulty": "Medium", "ac_rate": 60.02891996726984, "topics": ["array", "two-pointers", "greedy"], "url": "https://leetcode.com/problems/container-with-most-water/", "problem_html": "<p>You are given an integer array <code>height</code> of length <code>n</code>. There are <code>n</code> vertical lines drawn such that the two endpoints of the <code>i<sup>th</sup></code> line are <code>(i, 0)</code> and <code>(i, height[i])</code>.</p>\n\n<p>Find two lines that together with the x-axis form a container, such that the container contains the most water.</p>\n\n<p>Return <em>the maximum amount of water a container can store</em>.</p>\n\n<p><strong>Notice</strong> that you may not slant the container.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img alt=\"\" src=\"https://s3-lc-upload.s3.amazonaws.com/uploads/2018/07/17/question_11.jpg\" style=\"width: 600px; height: 287px;\" />\n<pre>\n<strong>Input:</strong> height = [1,8,6,2,5,4,8,3,7]\n<strong>Output:</strong> 49\n<strong>Explanation:</strong> The above vertical lines are represented by array [1,8,6,2,5,4,8,3,7]. In this case, the max area of water (blue section) the container can contain is 49.\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> height = [1,1]\n<strong>Output:</strong> 1\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>n == height.length</code></li>\n\t<li><code>2 <= n <= 10<sup>5</sup></code></li>\n\t<li><code>0 <= height[i] <= 10<sup>4</sup></code></li>\n</ul>\n", "hints": ["If you simulate the problem, it will be O(n^2) which is not efficient.", "Try to use two-pointers. Set one pointer to the left and one to the right of the array. Always move the pointer that points to the lower line.", "How can you calculate the amount of water at each step?"]}}, {"index": 8, "category": "coding_string", "question": "[Hard] LC #32 \u00b7 Longest Valid Parentheses \u2014 solve on leetcode.com", "why": "Topic tags: string, dynamic-programming, stack. AC rate 38.8%. Open the editor at https://leetcode.com/problems/longest-valid-parentheses/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "longest-valid-parentheses", "frontend_id": "32", "difficulty": "Hard", "ac_rate": 38.79922084932445, "topics": ["string", "dynamic-programming", "stack"], "url": "https://leetcode.com/problems/longest-valid-parentheses/", "problem_html": "<p>Given a string containing just the characters <code>'('</code> and <code>')'</code>, return <em>the length of the longest valid (well-formed) parentheses </em><span data-keyword=\"substring-nonempty\"><em>substring</em></span>.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n\n<pre>\n<strong>Input:</strong> s = "(()"\n<strong>Output:</strong> 2\n<strong>Explanation:</strong> The longest valid parentheses substring is "()".\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n\n<pre>\n<strong>Input:</strong> s = ")()())"\n<strong>Output:</strong> 4\n<strong>Explanation:</strong> The longest valid parentheses substring is "()()".\n</pre>\n\n<p><strong class=\"example\">Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> s = ""\n<strong>Output:</strong> 0\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>0 <= s.length <= 3 * 10<sup>4</sup></code></li>\n\t<li><code>s[i]</code> is <code>'('</code>, or <code>')'</code>.</li>\n</ul>\n", "hints": []}}, {"index": 9, "category": "coding_linked_list", "question": "[Hard] LC #25 \u00b7 Reverse Nodes in k-Group \u2014 solve on leetcode.com", "why": "Topic tags: linked-list, recursion. AC rate 66.1%. Open the editor at https://leetcode.com/problems/reverse-nodes-in-k-group/, talk through your approach, then sketch the algorithm here (whiteboard) or write a plain-English solution (typed).", "how_to_prepare": "Clarify constraints, propose a brute-force, refine to an optimal time/space, walk through 2 example inputs (including an edge case), then trace your final code by hand.", "leetcode": {"title_slug": "reverse-nodes-in-k-group", "frontend_id": "25", "difficulty": "Hard", "ac_rate": 66.08188830002686, "topics": ["linked-list", "recursion"], "url": "https://leetcode.com/problems/reverse-nodes-in-k-group/", "problem_html": "<p>Given the <code>head</code> of a linked list, reverse the nodes of the list <code>k</code> at a time, and return <em>the modified list</em>.</p>\n\n<p><code>k</code> is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of <code>k</code> then left-out nodes, in the end, should remain as it is.</p>\n\n<p>You may not alter the values in the list's nodes, only nodes themselves may be changed.</p>\n\n<p> </p>\n<p><strong class=\"example\">Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/10/03/reverse_ex1.jpg\" style=\"width: 542px; height: 222px;\" />\n<pre>\n<strong>Input:</strong> head = [1,2,3,4,5], k = 2\n<strong>Output:</strong> [2,1,4,3,5]\n</pre>\n\n<p><strong class=\"example\">Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2020/10/03/reverse_ex2.jpg\" style=\"width: 542px; height: 222px;\" />\n<pre>\n<strong>Input:</strong> head = [1,2,3,4,5], k = 3\n<strong>Output:</strong> [3,2,1,4,5]\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li>The number of nodes in the list is <code>n</code>.</li>\n\t<li><code>1 <= k <= n <= 5000</code></li>\n\t<li><code>0 <= Node.val <= 1000</code></li>\n</ul>\n\n<p> </p>\n<p><strong>Follow-up:</strong> Can you solve the problem in <code>O(1)</code> extra memory space?</p>\n", "hints": []}}]}