Leetcode: Sum of Square Numbers

## Blog link: https://code.dennyzhang.com/sum-of-square-numbers
## Basic Ideas: max(a, b) <= int(sqrt(c))
##              Then we convert it into a two-sum problem
##
##  Assumption: a, b can be 0
## Complexity: Time O(sqrt(n)), Space O(1)
class Solution(object):
    def judgeSquareSum(self, c):
        """
        :type c: int
        :rtype: bool
        """
        if c < 0:
            return False
        import math
        a, b = 0, int(math.sqrt(c))
        # a may equals b
        while a <= b:
            v = a*a + b*b
            if v == c:
                return True
            elif v > c:
                b -= 1
            else:
                a += 1
        return False