Reverse String II

Problem

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"

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

Solution

public class Solution {

    public String reverseStr(String s, int k) {
        if (s == null || k <= 1) return s;
        char[] chars = s.toCharArray();
        int start = 0, len = s.length();
        while (start < len - 1) {
            int end = Math.min(start + k - 1, len - 1); //Avoid index out of bounds 
            reverse(chars, start, end);
            start += 2 * k;
        }
        return  String.valueOf(chars);
    }

    private void reverse(char[] chars, int start, int end) {
        while (start < end) {
            char temp = chars[start];
            chars[start++] = chars[end];
            chars[end--] = temp;
        }
    }
}

Analysis

Typical reversing solution using two-pointer swap
The only difference between this problem and Reverse String is that we have a bound
Which means we need to have a helper method reverse() with given indices of two ends
We increase the start index every time finishing the reverse in k intervals by += 2 * k
Notice that we use Math.min() to avoid index out of bounds exception