69. Sqrt(x)（求平方根）（leetcode）

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.

分析：1、二分法的精髓为猜+判定，猜答案是什么，猜对了直接返回，猜错了修正答案。
sqrt*sqrt=x;
sqrt*mid=x;