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;

        }

    }

}