Remove extra spaces from string

#include<iostream>

using namespace std;

int main()
{
    string s, s1 = "", s2 = ""; getline(cin, s);

    // v1
    for (size_t i = 0; i < s.length(); ++i)
    {
        if ((s[i] != ' ') || ((s[i] == ' ') && (s1 != "") && (s1[s1.length()-1] != ' '))) s1 += s[i];
    }

    // more readable v2
    for (auto c : s)
    {
        if ((c != ' ') || ((c == ' ') && (s2 != "") && (s2.back() != ' '))) s2 += c;
    }

    cout << "Input string: " << s << endl << "Output string v1: " << s1 << endl << "Output string v2: " << s2 << endl;

    return 0;
}