分析所有情况即可，注意判断是否越界

#include<bits/stdc++.h>
using namespace std;
int main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	int n,q;string s;cin>>n>>q>>s;
	s=" "+s;int ans=0;
	for(int i=1;i<=n-2;i++)
		if(s[i]=='A'&&s[i+1]=='B'&&s[i+2]=='C')
			ans++;
	while(q--)
	{
		int x;char c;
		cin>>x>>c;
		if(x<=n-2&&s[x]=='A'&&s[x+1]=='B'&&s[x+2]=='C'&&c!='A')ans--;
		if(x>1&&s[x]=='B'&&s[x-1]=='A'&&s[x+1]=='C'&&c!='B')ans--;
		if(x>2&&s[x]=='C'&&s[x-1]=='B'&&s[x-2]=='A'&&c!='C')ans--;
		if(x<=n-2&&c=='A'&&s[x+1]=='B'&&s[x+2]=='C'&&s[x]!='A')ans++;
		if(x>1&&c=='B'&&s[x-1]=='A'&&s[x+1]=='C'&&s[x]!='B')ans++;
		if(x>2&&c=='C'&&s[x-1]=='B'&&s[x-2]=='A'&&s[x]!='C')ans++;
		s[x]=c;cout<<ans<<'\n';
	}
	return 0;
}