sqrtx

题目：

Implementint sqrt(int x).
Compute and return the square root of x.

程序：

class Solution {
public:
    int sqrt(int x) {//思路用二分法
        if (x < 2)
            return x;
        int left = 1, right = x / 2;    //右端从x/2开始
        int mid, last_mid;

        while (left <= right) {
            mid = left + (right - left) / 2;
            if (x / mid > mid) { //不用x > mid * mid 会溢出
                left = mid + 1;
                last_mid = mid;
            }
            else if (x / mid < mid)
                right = mid - 1;
            else
                return mid;
        }
        return last_mid;
    }
};

点评：

采用二分法，两端逼近，在做连续序列或者数组类题目时，可用二分法减少时间复杂度