参考答案：

#include<iostream>
#include<string>
using namespace std;
string s;
int main()
{
    int n, m;
    cin >> n >> m;
    cin >> s;
    while (m--)
    {
        int x, y;
        char q, w;
        cin >> x >> y;
        cin >> q >> w;
        for (int i = x - 1; i < y; ++i) if (s[i] == q) s[i] = w;
    }
    cout << s << '\n';
    return 0;
}