题目描述：

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.

Example:

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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]

按照题目要求翻转字符串即可。

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class Solution {
public:
    string reverseStr(string s, int k) {
        int k2 = k << 1;
        int i;
        for (i = 0; i < s.length(); i += k2) {
            if (i + k <= s.length())
                reverseStr(s, i, i + k);
            else
                reverseStr(s, i, s.length());
        }
        return s;
    }
    
    void reverseStr(string &s, int left, int right) {
        for (int i = 0; left + i < right - i - 1; i++) {
            swap(s[left + i], s[right - i - 1]);
        }
    }
};