Pytorch on Rui's Blog

Leetcode 541 - Reverse String II

Mon, 19 Jan 2026 23:12:00 -0800

Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.

If there are fewer than k characters left, reverse all of them.

If there are less than 2k but greater than or equal to k characters, reverse the first k characters and leave the other as original.

Mon, 19 Jan 2026 09:00:00 -0800

Write a function that reverses a string. The input string is given as an array of characters s.

You must do this by modifying the input array in-place with O(1) extra memory.

Example 1:

Input: s = [“h”,“e”,“l”,“l”,“o”] Output: [“o”,“l”,“l”,“e”,“h”] Example 2:

Input: s = [“H”,“a”,“n”,“n”,“a”,“h”] Output: [“h”,“a”,“n”,“n”,“a”,“H”]

Opposite Direction Pointers: One pointer starts at the beginning, and the other starts at the end.

Both pointers move inward simultaneously with the same step size until they cross each other (left > right), at which point the traversal ends.

Sun, 18 Jan 2026 22:21:00 -0800

Given an array nums and a value val, you need to in-place remove all instances of that value and return the new length of the array.

Do not use extra space for another array; you must do this by modifying the input array in-place with O(1) extra space.

The order of elements can be changed. You do not need to consider the elements beyond the new length.