#include<iostream>
using namespace std;

int main(){
    int flag = 0;
    string s1, s2;
    getline(cin, s1);
    for(int i = 0; s1[i]; ++i){
        if(s1[i] == ' ' && flag == 0){
            flag = 1;
            s2 = s2 + s1[i];
        }
        else if(s1[i] != ' ' && flag == 1){
            s2 = s2 + s1[i];
            flag = 0;
        }
        else if(s1[i] != ' ')
            s2 = s2 + s1[i];
    }

    cout << s2 << endl;
    return 0;
}

用cin或者scanf自动过滤空格

#include<iostream>
using namespace std;

int main(){
    string s;
    while(cin >> s)
        cout << s << " ";
    return 0;
}