字符串弱等于 （以及substr函数用法）

#include <iostream>

using namespace std;

bool f(string a,string b){
    if(a == b) return true;
    else if(a.length() == b.length()&&a.length()%2 == 0){
        string a1 = a.substr(0,a.length()/2);
        string a2 = a.substr(a.length()/2,a.length());
        string b1 = b.substr(0,b.length()/2);
        string b2 = b.substr(b.length()/2,b.length());
        return f(a1,b1)&&f(a2,b2)||f(a1,b2)&&f(a2,b1);
    }
    else return false;
}
int main(){
    string a,b;
    cin>>a>>b;
    if(f(a,b)) cout<<"YES";
    else cout<<"NO";
    return 0;
}