不会find函数怎么办

没关系你会用substr

#include<bits/stdc++.h>
using namespace std;
int main(){
    string a,b;
    cin>>a>>b;
    if(a==b){
        cout<<a<<" is substring of "<<b<<endl;
        return 0;
    }
    else if(a.size()==b.size()){
        cout<<"No substring\n";
        return 0;
    }
    string str="";
    if(a.size()<b.size()){
        for(int i=0;i<b.size()-a.size();i++){
            str=b.substr(i,a.size());
            if(str==a){
                cout<<a<<" is substring of "<<b<<endl;
                return 0;
            }
        }
    }
    else{
        for(int i=0;i<a.size()-b.size();i++){
            str=a.substr(i,b.size());
            if(str==b){
                cout<<b<<" is substring of "<<a<<endl;
                return 0;
            }
        }
    }
    cout<<"No substring\n";
    return 0;
}

写的有点啰嗦，可以删掉一些不必要的判断