B2135传送门 找不到问题

#include<bits/stdc++.h>
#include<iostream>
using namespace std;
int main(){
	string s1,s2,s3;
	getline(cin,s1);
	cin>>s2>>s3;
	int a=s1.length();
	string s="";
	s1+=' ';
	for(int i=0;i<=a;i++){
		if(s1[i]==' '){
			if(s==s2) cout<<s3<<" ";
			else cout<<s<<" ";
			s="";
		}
		else{
			s+=s1[i];
		}
	}
	cout<<endl;
	return 0;
}