问题描述：

第一种方法：对撞指针

#include <iostream>

using namespace std;

/// Two Pointers
/// Time Complexity: O(n)
/// Space Complexity: O(1)
class Solution {
public:
    string reverseString(string s) {

        int i = 0, j = s.size() - 1;
        while(i < j){
            swap(s[i], s[j]);
            i ++;
            j --;
        }

        return s;
    }
};


int main() {

    cout << Solution().reverseString("hello") << endl;

    return 0;
}

 第二种方法：

class Solution {
public:
    string reverseString(string s) {
        int start = 0;
        int end = s.length() - 1;
        char ch = 0;
        
        for (; start < end; start++, end--)
        {
            ch = s[start];
            s[start] = s[end];
            s[end] = ch;
        }
        
        return s;
    }
};