非子串的子序列

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n, q;
    cin >> n >> q;
    string s;
    cin >> s;
    bool flag[n] = {true}, flag2[n] = {true};
    int a, b, p, num = 0, c = 0;
    while (q--)
    {
        cin >> a >> b;
        a -= 1;
        b -= 1;
        p = b - a + 1;
        string t;
        t = s.substr(a, p);
        // cout << s << endl;
        // cout << t << endl;
        for (int i = 0; i < a; i++)
        {
            if (s[i] == t[0])
            {
                num++;
                // cout << i << "&&&&";
                break;
            }
        }
        for (int i = b + 1; i < n; i++)
        {
            if (s[i] == t[p - 1])
            {
                num++;
                // cout << i << "&&&&";
                break;
            }
        }
        if (num >= 1)
        {
            cout << "YES" << endl;
        }
        else
        {
            cout << "NO" << endl;
        }
        num = 0;
    }
}