LeetCode 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:

1 public class Solution {
2     public boolean isPerfectSquare(int num) {
3         long l = 1;
4         long r = num;
5
6         while(l <= r){
7             long mid = l+(r-l)/2;
8             long square = mid*mid;
9             if(square == num){
10                 return true;
11             }else if(square > num){
12                 r = mid-1;
13             }else{
14                 l = mid+1;
15             }
16         }
17         return false;
18     }
19 }

类似Sqrt(x).