541. Reverse String II

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.

Example:

Input: s = "abcdefg", k = 2 Output: "bacdfeg"

Restrictions:

1. The string consists of lower English letters only.
2. Length of the given string and k will in the range [1, 10000]

//每2k个字符里，翻转前k个 class Solution {     public String reverseStr(String s, int k) {         char[] arr = s.toCharArray();         int n = arr.length;         int i = 0;         while (i < n) {             int j = Math.min(i + k - 1, n - 1); //i的值确定后，j的值为i + k - 1，要判断是否超出数组(和n - 1比较)             swap(arr, i, j);             i += 2 * k;         }         return new String(arr);        //return String.valueOf(arr);     }          private void swap(char[] arr, int l, int r) {         while (l < r) {             char temp = arr[l];             arr[l++] = arr[r];             arr[r--] = temp;         }     } }