各路大佬行行好，帮我看看这是怎么回事T_T

#include<bits/stdc++.h>
using namespace std;
int summery(const string& str, const string& sub)
{
	int num = 0;
	for (int i = 0; (i = str.find(sub, i)) != -1; num++, i++);
	return num;
}
vector<string> w;
int main(){
	string s;
	cin>>s;
	string x="";
	for(auto &i:s){
		if(islower(i)){
			x+=i;
		}
		if(i=='P'){
			w.push_back(x);
		}
		if(i=='B'){
			x.pop_back();
		}
	}
	int n;
	cin>>n;
	for(int i=0;i<n;i++){
		int a,b;
		cin>>a>>b;
		cout<<summery(w[b-1],w[a-1])<<'\n';
	}
	return 0;
}

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