不用find函数也可以解

不用函数也可以啊，速度还挺快的，就是内存消耗的有点多....

#include<iostream>
using namespace std;

int main(){
    string s1 , s2;
    cin >> s1 >> s2;
    int l1 = s1.length() , l2 = s2.length() ;
    int point = 0;
    for (int i = 0; i <= l2 - 1; i++){
        if (s2[i] == s1[point]){
            point ++;
        }else if(s2[i] == s1[0]){
            point = 1;
        }else{
            point = 0;
        }
        if (point == l1){
            cout << s1 << " is substring of " << s2;
            break;
        }
    }
    if (point != l1){
        cout << "No substring" ;
    }
    return 0;
}