解法1

直接找对应单词

#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;
}

解法2

正则匹配

#include <iostream>
#include <regex>
using namespace std;
int main(){
    string s,a,b;
    getline(cin,s);
    cin>>a>>b;
    cout<<regex_replace(s,regex("\\b" + a + "\\b"),b)<<endl;
    return 0;
}

解法3

sstream

#include <iostream>
#include <sstream>

using namespace std;

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

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

    stringstream ssin(s);
    string str;
    while (ssin >> str)
    // 遇到空格停止，所以可以分割单词
        if (str == a) cout << b << ' ';
        else cout << str << ' ';

    return 0;
}