I can solve it in O(1)!!!
you must solve equation

n - we have
x - we must find

if (x * (x + 1) / 2)+ 1 == n cout << 1
else cout << 0;

but if you want you can find x in o(log n) - by binary search
or in o(1) if you solve equation!!
Good luck in solving this problem=)
Sorry for me bad english=(

Edited by author 28.11.2010 23:28