Find square root of an integer without using built in functions

Problem:
Find square root of an integer without using built in functions.

Solution:
Provided we don't know or can't recall any math: square roots of integer numbers are "sorted" in a way that the grater the number, the greater its square root is. So we can use binary search starting with the interval (0; n) and set x = (n - 0) / 2, continuing dividing the interval until we get | x*x - n | < epsilon, where the epsilon is some very small number (e.g. 0.0000000001).

Complexity:

time - O( ??? ) - I'd speculate something like O (log n / epsilon). log n comes from using the binary search, and obviously, the higher the required precision, the longer the search takes, hence the (1 / epsilon) multiplier.
space - O(1)

Links and credits:
http://www.careercup.com/question?id=6657802751705088