Spaces.cpp

// g++ -O3 Spaces.cpp -o Spaces --std=c++14 -march=native -g; time ./Spaces

#include <iostream>
#include <string>

using namespace std;

static string InjectSpaces(const string& s) {
    string result(2*s.length() - 1, ' ');
    const auto length = s.length();
    for (auto i = 0; i < length; ++i) {
        result[2*i] = s[i];
    }
    return result;
}

int main() {
    string s = "1234567890";
    string result;
    for (size_t i = 0; i < 200000000; ++i) {
        result = InjectSpaces(s);
    }
    cout << result << endl;

    return 0;
}

👍