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]

public class Solution {public  String reverse(String s) {if (s == null  || s.length() == 0) {return null;}char[] chars = s.toCharArray();int length = chars.length;int start = 0, end = length - 1;while (start < end) {char tmp = chars[start];chars[start++] = chars[end];chars[end--] = tmp;}String result = "";for (char c : chars) result += c;return result;}public  String reverseStr(String s, int k) {if (null == s || s.length() == 0)return null;int length = s.length();int one = 1;int count = length % k == 0 ? length / k : length / k + 1;String result = "";for (int i = 1; i <= count; i++) {if (one % 2 == 1)result += reverse(s.substring(k * (i -1),  (k * i > length ? length : k * i)));else result += s.substring(k * (i -1),  (k * i > length ? length : k * i));one++;}return result;}
}

 

先把问题化为一小步，分治的思想，比如我们先实现字符串的反转功能，然后再更具条件来实现，哪些子字符串需要反转