#include <iostream>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

vector<double> num;

int main()
{
    double now;
    while(!scanf("%lf", &now)) {
        num.push_back(now);
    }

    for (unsigned i = num.size()-1; i>=0; --i) {
        printf("%.4lf\b", sqrt(num[i]));
    }
    return 0;
}

-- use a array[200000] got AC, but why vector failed?

If the input is empty, "num.size()-1" is a huge positive integer