69. Sqrt(x)

问题描述：

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.

解题思路：

这道题目可以用二分法来不断猜测根的值并进行逼近。

代码：

class Solution {
public:
    int mySqrt(int x) {
        if(x <= 1)
            return x;
        int left = 0;
        int right = x;
        while(left < right){
            int mid = (left + right)/2;
            if(x / mid >= mid)
                left = mid+1;
            else
                right = mid;
        }
        return right-1;
    }
};