# 541. 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"
``````

Restrictions:

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

### Solutions:

``````public class Solution {
public String reverseStr(String s, int k) {
if (s == null || s.length() == 0 || k == 0) {
return "";
}
int i = 0;
StringBuilder sb = new StringBuilder(s);
while (i < s.length()) {
int left = i;
int right = Math.min(i + k - 1, s.length() - 1);
while (left < right) {
char a = sb.charAt(left);
char b = sb.charAt(right);
sb.setCharAt(left, b);
sb.setCharAt(right, a);
left ++;
right --;
}
i = i + 2 * k;
}
return sb.toString();
}
}
``````