Valid Perfect Square

Given a positive integer num, write a function that returns True if num is a perfect square else False.
Note: Do not use any built-in library function such as sqrt.

Example 1:

Input: 16

Output: true

Binary Search Approach - O(log n)

public boolean isPerfectSquare(int num) {
    long left = 1;
    long right = num;
    while (left<= right) {
        long mid = left + (right -left) /2;
        if (mid * mid == num)
            return true;
        if (mid * mid < num) {
            left = mid + 1;
        } else
            right = mid - 1;
    }
    return false;
}

Square Root Approach - O(sqrt(n))
We have observed the equation and find some similarities, let's discuss with an example.
Input: 16
1 + 3 + 5 + 7 = 16
So 1 + 3 + 5 + 7 + 9 + 11 + ... + (2n -1)
= (2(n-1) + 1) n/2  = n*n

public boolean isPerfectSquare(int num) {
    int i = 1;
    while (num > 0) {
        num -= i;
        i += 2;
    }
    return num == 0;
}