leetcode_367. 有效的完全平方数 && leetcode_69. x 的平方根

二分查找  每次判断mid*mid和num 的大小，选择缩小区间 

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

也是同样的思路：二分查找

class Solution {
public:
    bool isPerfectSquare(int num) {
        int left = 0;
        int right = num;
        while (left <= right) {
            int mid = (left + right) / 2;  
            if ((long long)mid*mid > num) {
                right = mid - 1;
            }else if ((long long)mid*mid < num) {
                left = mid + 1;
            }
            else {
                return true;
            }
        }
        return false;
    }
};