题目比较简单

class Solution {
public:
    /**
     * 反转字符串
     * @param str string字符串 
     * @return string字符串
     */
    string solve(string str) {
        // write code here
        int i = str.length() - 1;
        int j = 0;
        while(i>j) swap(str[i--], str[j++]);
        return str;
    }
};