技術標籤：LeetCode演算法與資料結構mathhash table

Topic

• Math
• Hash Table

Description

https://leetcode.com/problems/happy-number/

Write an algorithm to determine if a number n is happy.

A happy number is a number defined by the following process:

• Starting with any positive integer, replace the number by the sum of the squares of its digits.
• Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
• Those numbers for which this process ends in 1 are happy.

Return true if n is a happy number, and false if not.

Example 1:

Input: n = 19
Output: true
Explanation:
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1
 

Example 2:

Input: n = 2
Output: false

Constraints:

• 1 <= n <= 2³¹ - 1

Analysis

方法一：關鍵用雜湊表快取中間計算結果。

方法二：快慢指標查環思路。

Submission

import java.util.HashSet;
import java.util.Set;

public class HappyNumber {

	// 方法一：
	public boolean isHappy1(int n) {
		if (n < 1) return false;
		Set<Integer>
 
 cache = new HashSet<>();

		while (!cache.contains(n)) {
			cache.add(n);
			int sum = 0;
			while (n > 0) {
				int temp = n % 10;
				sum += temp * temp;
				n /= 10;
			}
			if ((n = sum) == 1) return true;
		}
		return false;
	}

	// 方法二：
	public boolean isHappy2(int n) {
		int slow, fast;
		slow = fast = n;
		do {
			slow = digitSquareSum(slow);
			fast = digitSquareSum(fast);
			if (fast == 1) return true;
			fast = digitSquareSum(fast);
			if (fast == 1) return true;
		} while (slow != fast);

		return false;
	}

	private int digitSquareSum(int n) {
		int sum = 0, tmp;
		while (n > 0) {
			tmp = n % 10;
			sum += tmp * tmp;
			n /= 10;
		}
		return sum;
	}

}

Test

import static org.junit.Assert.*;
import org.junit.Test;

public class HappyNumberTest {

	@Test
	public void test() {
		HappyNumber obj = new HappyNumber();
		
		assertTrue(obj.isHappy1(19));
		assertFalse(obj.isHappy1(2));
		
		assertTrue(obj.isHappy2(19));
		assertFalse(obj.isHappy2(2));
	}
}