#include <iostream>
#include <stdio.h>
#include <math.h>
using namespace std;
int main(){
double a[100000];
long long n,i=0;
while (scanf("%I64d", &n) != EOF)
{
    a[i] = sqrt((double)n);
    i++;
}
for (int x=i-1;x>=0;x--)
    printf("%0.4f\n",a[x]);
return 0;
}

If I put range of array to 300000, i have "Crash (stack overflow)" on first test. What's wrong?
P.S Sorry for my bad English

All variables inside functions is a dynamical variables, they always located in stack. By default stack is quite small. To avoid stack overflow move your array outside main function:
double a[1000000];
int main() {
}
You can also increase stack size, for that see help on C++.
P.S. Sorry for bad english too :)

Thank you very much