69.x的平方根

class Solution:
    def mySqrt(self, x: int) -> int:
        if x < 2:
            return x

        left, right = 1, x//2
        while left <= right:
            mid = left + (right - left) // 2
            if mid > x / mid:
                right = mid - 1
            else:
                left = mid + 1
        return left - 1