LeetCode 367. Valid Perfect Square

原题链接在这里：https://leetcode.com/problems/valid-perfect-square/

题目：

Given a positive integer num, write a function which returns True if num is a perfect square else False.

Note: Do not use any built-in library function such as sqrt.

Example 1:

Input: 16
Returns: True

Example 2:

Input: 14
Returns: False

题解：

Binary Search, 看mid*mid == num, 若等于就是perfect square了. 若是大于就在[l, mid-1]区间找. 若是小于就在[mid+1, r]区间找.

注意overflow, 若num = Integer.MAX_VALUE, 那么mid*mid肯定overflow了. 所以mid 和 square都是long型.

Time Complexity: O(logn), n =num.

Space:O(1).

AC Java:

public class Solution {
    public boolean isPerfectSquare(int num) {
        long l = 1; 
        long r = num;
        
        while(l <= r){
            long mid = l+(r-l)/2;
            long square = mid*mid;
            if(square == num){
                return true;
            }else if(square > num){
                r = mid-1;
            }else{
                l = mid+1;
            }
        }
        return false;
    }
}

类似Sqrt(x).