怎么没人发题解？

#include<bits/stdc++.h>
using namespace std;

int main() {
	string s;
    getline(cin, s);
    string a, b;
    cin >> a >> b;
    s += ' ';
    int en = s.find(" "), st = 0;
    while(en != -1) {
        string t = "";
        for (int i = st; i < en; i++)
            t += s[i];
        if (t == a) cout << b << " ";
        else cout << t << " ";
        st = en + 1, en = s.find(" ", st);
    }
	return 0;
}