28-69. Sqrt(x)

题目描述：

Implement int sqrt(int x).

Compute and return the square root of x, where x is guaranteed to be a non-negative integer.

Since the return type is an integer, the decimal digits are truncated and only the integer part of the result is returned.

Example 1:

Input: 4
Output: 2

Example 2:

Input: 8
Output: 2
Explanation: The square root of 8 is 2.82842..., and since 
             the decimal part is truncated, 2 is returned.

代码实现：

 1 class Solution(object):
 2     def mySqrt(self, x):
 3         """
 4         :type x: int
 5         :rtype: int
 6         """
 7         l, r = 0, x
 8         while l <= r:
 9             mid = l + (r-l)//2
10             if mid * mid <= x < (mid+1)*(mid+1):
11                 return mid
12             elif x < mid * mid:
13                 r = mid
14             else:
15                 l = mid + 1

分析：binary search