Interview Check If n Is A Perfect Square

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;
		}
	}
}

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