python写算法题：leetcode: 69. Sqrt(x)

class Solution(object):
    def mySqrt(self, x):
        """
        :type x: int
        :rtype: int
        """
        if x==0:
            return 0
        n=1
        step=x
        while step>=4:
            step>>=2
            n<<=1
        step=n
        while step>0:
            if (n+step)*(n+step)<=x:
                n+=step
            step>>=1
                
        return n