find max n such that s <= LONG_MAX, for s=1+2+3+...+n (O(1) algorithm)

n.c

#include <stdio.h>
#include <limits.h>
#include <math.h>

int main()
{
    double nn=(1+sqrt(1+4.0*LONG_MAX*2.0))/2.0;
    long int n= nn, s=n/2*(n-1),t;
    printf("%lf %lf\n", nn, log(nn)/log(2.0));
    printf("%ld %ld %ld\n", n, s, s+n+1);
}