不行，老搞错🤣

终于对了🤣

#include<bits/stdc++.h>
using namespace std;
int main(){
    string s,s2;
    int n;
    char d;
    bool f = true;
    cin>>n>>s>>d;
    for (int i = 0;i < n;i++){
        if (f){
            if (s[i] + '0' < d + '0'){
                s2 += d;
                f = false;
            }
        }
        s2 += s[i];
    }if (f){
        s2 += d;
    }cout<<s2;
    return 0;
}```