367. Valid Perfect Square

367. Valid Perfect Square

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: 16Returns: True

Example 2:

Input: 14 Returns: False

完全平方数是一系列奇数之和，例如：

1 = 1 4 = 1 + 3 9 = 1 + 3 + 5 16 = 1 + 3 + 5 + 7 25 = 1 + 3 + 5 + 7 + 9 36 = 1 + 3 + 5 + 7 + 9 + 11 …. 1+3+…+(2n-1) = (2n-1 + 1)n/2 = n*n

public class Solution { public boolean isPerfectSquare(int num) { int i = 1; while (num > 0) { num -= i; i += 2; } return num == 0; }}

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