力扣541. 反转字符串 II

class Solution {
public:
    void reverse_k(string& s, int start, int end) {
        for(int i = start, j = --end; i < j; i++, j--) {
            char tmp = s[i];
            s[i] = s[j];
            s[j] = tmp;
        }
    }

    string reverseStr(string s, int k) {
        for(int i = 0; i < s.size(); i += 2 * k) {
            if(i + k <= s.size()) {
                reverse_k(s, i, i + k);
            } else {
                reverse_k(s, i, s.size());
            }
        }
        return s;
    }
};