各位大佬帮帮忙

#include <bits/stdc++.h>

using namespace std;

int n, k;
string s;
int a, b;
char c, d;

int main()
{
	
	
	cin >> n >> k;
	
	cin >> s;

	for (int i = 0; i < k; ++i)
	{
		scanf("%d %d %c %c",&a, &b, &c, &d);
		
		a--, b--;
		
		for (int j = a; j <= b; ++j)
		{
			if (s[j] == c)
			{
				s[j] == d;
			}
		}
	}
	
	for (int i = 0; i < s.size(); ++i)
	{
		cout << s[i];
	}
	
	return 0;
}