第一种

/*
#include<iostream>
#include<string>

using namespace std;

int main(){
    string s,a,b;
    getline(cin,s);
    cin >> a >> b;

    for(int i=0;i>s.size();i++){
        string word;
        int j=i;

        while(true){//检测是否是一个单词，标记是空格
            if(j<s.size() && s[j]!=' '){
                j++;
                word = word + s[j];
                cout<<word<<endl;
            }else{
                break;
            }
        }
        //while(j < s.size() && s[j] != ' ') word = word + s[j], j ++;
        i = j;

    }

    return 0;
}*/
#include <iostream>
#include <sstream>

using namespace std;

int main()
{
    string s, a, b;

    getline(cin, s);
    cin >> a >> b;

    for (int i = 0; i < s.size(); i ++)
    {
        string word;
        int j = i;
        while(j < s.size() && s[j] != ' ') word = word + s[j], j ++;

        if (word == a) cout << b << " ";
        else cout << word << " ";
        i = j;
    }

    return 0;
}

第二种stringstresm

int main(){

    string str;
    getline(cin,str);

    string a,b;
    cin >> a >> b;

    stringstream ssin(str);   //将字符串初始化成字符串流

    string temp;
    while(ssin >> temp){
        if(temp == a){
            cout<<b << " ";
        }else{
            cout<< temp << " ";
        }
    }


    return 0;
}