过了必关！！！

#include <bits/stdc++.h>
using namespace std;
//fixed << setprecision()
string a, b;

int main(){
	ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
	cin >> a >> b;
	int t = max(a.size(), b.size());
	for (int i = 0; i < t; i++)
	{
		if (a.size() < b.size())
		{
			if (b[i] == a[0])
			{
				int n = 0;
				for (int j = i, s = 0; j < a.size(); j++, s++)
				{
					if (b[i] != a[s])
					{
						n++;
						break;
					}
				}
				if (n == 0)
				{
					cout << a << " is substring of " << b;
					return 0;
				}
			}
		}
		else
		{
			if (a[i] == b[0])
			{
				int n = 0;
				for (int j = i, s = 0; j < b.size(); j++, s++)
				{
					if (a[i] != b[s])
					{
						n++;
						break;
					}
				}
				if (n == 0)
				{
					cout << b << " is substring of " << a;
					return 0;
				}
			}
		}
	}
	cout << "No substring";
	return 0;
}

本人蒟蒻勿喷