Description

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

解法：

对于一个长度小于k的字符串来说很简单，直接逆序就可以。如果长度在K和2K中间，把前K个取反，K-2K不变就可以了。所以把题目可以简化为这个字符串有多少个2K长度的字符串，再对每一个2K长度的字符串进行处理即可。

public String reverseStr(String s, int k) { if(s.length() <= k) return new StringBuffer(s).reverse().toString(); else if(s.length() <= 2*k) return new StringBuffer(s.substring(0, k)).reverse().toString() + s.substring(k); String result = ""; while(s.length() > 2*k) { result += reverseStr(s.substring(0,2*k),k); s = s.substring(2*k); } result += reverseStr(s,k); return result; }