使用sstream

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main ()
{
    string s1, s2, s3;
    getline(cin, s1);
    cin >> s2 >> s3;

    stringstream ssin(s1);       // 将字符串s1当作输入流
    string str;
    while (ssin >> str)
    {
        if (str == s2) str = s3;
        cout << str << ' ';
    }

    return 0;
}

方法二，使用string a[]

使用字符串数组

#include <iostream>
#include <string>

using namespace std;

string a[100];
int main ()
{
    string s2, s3;
    int n = 0;
    while (cin >> a[n ++])
    {
        char x = getchar();
        if (x == '\n')
        break;
    }

    cin >> s2 >> s3;
    for (int i = 0; i < n; i ++)
    {
        if (a[i] == s2) a[i] = s3;
        cout << a[i] << ' ';
    }

    return 0;
}