Binary Search:69. Sqrt(x)

这道题求x的平方根。

注意用long long。写的时候最后一个return right总是写不对。因为如果x不能得到整数平方根，那么就向下取整。由于在while结束时，left会指向较大的整数，right指向较小的整数，所以返回right

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