069._sqrt(x).md

69. Sqrt(x)

题目: https://leetcode.com/problems/sqrtx/

难度:

Medium

思路：

一看，觉得很容易，一写，超时：

class Solution(object):
    def mySqrt(self, x):
        """
        :type x: int
        :rtype: int
        """
        i = 0
        while i * i <= x :
        	if i * i == x:
        		return i 
        	elif i * i < x and (i+1) * (i+1) > x:
        		return i
        	i += 1

看一眼tag， binary search，难道从x/2之类的开始搜起来？话说还想到求sqrt有个🐂的牛顿法？

莫名其妙过了的代码：

class Solution(object):
    def mySqrt(self, x):
        """
        :type x: int
        :rtype: int
        """
        if x == 1 : return 1
        if x == 0 : return 0
        l, r =  0, x - 1
        while l <= r:
        	mid = l + ((r - l) >> 2)
        	if mid * mid <= x and (mid+1)*(mid+1) > x:
        		return mid
        	elif mid * mid > x:
        		r = mid - 1 
        	else:
        		l = mid + 1 

或者

class Solution(object):
    def mySqrt(self, x):
        """
        :type x: int
        :rtype: int
        """
        if x == 1:
            return 1
        if x == 0:
            return 0
        l, r = 0, x-1
        while l <= r:
            mid = l + ((r - l) >> 2)
            if (mid * mid - x == 0):
                return mid
            elif (mid * mid - x > 0):
                r = mid - 1
            else:
                l = mid + 1
        return r

牛顿法

参见wikipedia，to be done：自己推导一遍 https://zh.wikipedia.org/wiki/牛顿法

class Solution(object):
    def mySqrt(self, x):
        """
        :type x: int
        :rtype: int
        """
        res = 1.0
        while abs(res * res - x) > 0.1:
            res = (res + x / res) / 2
        return int(res)