n! = omega(2^n)
n! = n * (n-1) * (n-2) * .......2 * 1 > 2 * 2 * 2* 2* ...... * 2 (n times) ; n > 2, (n-1) > 2 , (n-3) >2
so, n! = omega(2^n)
n! = n * (n-1) * (n-2) * .......2 * 1 < n * n * n *n *n (n times) ; n = n , n-1 <n ; n-2 <n ; n-3 < n
so , n! = big-O( n^n)