69. Sqrt(x)

求平方根，这一题discussion中有一道很好的二分查找的办法，代码实现如下：

class Solution(object):
    def mySqrt(self, x):
        """
        :type x: int
        :rtype: int
        """
        if x == 0:
            return 0
        left = 1
        right = pow(2, 31) - 1
        while True:
            mid = (left + right) / 2
            if mid >  x / mid:
                right = mid - 1
            else:
                if (mid + 1) > x / (mid + 1):
                    return mid
                else:
                    left = mid + 1