Perfect Square - Binary Search

You can use binary search to find the square root. Search for an integer $x$ in the range $[1, num]$ such that $x^2 = num$.

For each midpoint $mid$, compute $mid \times mid$. If it equals $num$, you found it. If it's less, search the right half. If it's more, search the left half.

This runs in $O(\log num)$ time because you halve the search space at each step. No square root function needed.