这会有问题？？？

#include<iostream>
#include<string.h>
#include<algorithm>
#include<vector>
#include<math.h>
#include<iomanip>
using namespace std;

int main()
{
    string s,str="",s1,s2;
    getline(cin,s);
    int i=0,n=0;
        cin>>s1;
        cin>>s2;
    while(i<s.length())
    {
        while(s[i]!=' '&&i<s.length())
        {
            str+=s[i];
            i++;
        }
        n++;
        i++;
        if(str==s1)
            cout<<s2<<" ";
        else
            cout<<str<<" ";
        str="";
    }
	return 0;
}