标签是递归
我来写个递归吧

#include<iostream>

using namespace std;

void reverse(string s, int i)
{
    if(i <= s.size()){
        reverse(s, i+1);
        printf("%c", s[i-1]);
    }

}

int main()
{
    string s;
    cin >> s;
    reverse(s, 0);
    return 0;
}