问题描述：
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.
示例：
Input: s = "abcdefg", k = 2 Output: "bacdfeg"问题分析：
题目要求每2k个字符中，翻转前k个，实现起来比较简单，直接上代码：
class Solution { public: string reverseStr(string s, int k) { int len = s.length(); int i = 0; for(; i + 2 * k < len ; i += 2 * k) { reverse(s.begin() + i, s.begin() + i + k); } len = len - i; if(len <= k) { reverse(s.begin() + i, s.end()); } else if(len <= 2 * k) { reverse(s.begin() + i, s.begin() + i + k); } return s; } };