循环语句 + 字符串操作

#include<bits/stdc++.h>
using namespace std;
int main(){
    string a="4567",b="123";
    string c;
    int n;
    cin>>n;
    while(c.length()<=n){
        c=b+a;
        a=b;
        b=c;
    }
    cout<<c[n-1];
    return 0;
}