Check if a given number is a perfect square with only addition or substraction operation.

eg. 25 returns true; 19 returns false.

Perfect square number 有一个特性，比如0,1,4,9,16,25 他们之间的间隔分别是1,3,5,7,9，每次间隔都是i+2.

所以每次往下减i, i 更新成i+2. 看最后结果是否为0即可。

import java.util.*;

public class isPerfectSquare{

	public static void main(String [] args){

		int num1 = 0;

		int num2 = 1;

		int num3 = 25;

		int num4 = 19;

		System.out.println(isPerfectSquare(num1));

		System.out.println(isPerfectSquare(num2));

		System.out.println(isPerfectSquare(num3));

		System.out.println(isPerfectSquare(num4));

	}

	private static boolean isPerfectSquare(int n){

		//1,4,9,16,25

		//1+3+5+7+9

		if(n<0){

			return false;

		}

		int i = 1;

		while(n>0){

			n-=i;

			i+=2;

		}

		if(n == 0){

			return true;

		}else{

			return false;

		}

	}

}