几种办法：
1）交换字符法

class Solution {
public:
    string reverseString(string s) {
        int i = 0;
        int len = s.size();
        string res = s;
        while (i <= (len-1) / 2){
            char temp = res[i];
            res[i] =  res[len-i-1];
            res[len-i-1]= temp;
            i += 1;
        }
        return res;
        
    }
};

2）栈的方法

class Solution {
public:
    string reverseString(string s) {
        stack<char>temp;
        for (int i = 0; i<s.size();++i){
            temp.push(s[i]);
        }
        string res;
        while(!temp.empty()){
            char t;
            t = temp.top();
            res = res + t;
            temp.pop();
        }
        return res;
        
    }
};

报了Memory Limit Exceeded
的错误
把res = res + t 改成了res.push_back(t)