[LeetCode] 202. Happy Number

Problem

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:
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1

Example 2:

Input: n = 2
Output: false

Constraints:

• 1 <= n <= 2^31 - 1

Solution

Logic

1. 자료구조 Set을 선언한다

2. 정수 n을 Set에 삽입한다.

3. n의 각 자리수를 제곱한 뒤, 더하여 새로운 n을 만들고 2번 과정을 다시 수행한다.

4. n이 Set에 존재한다면 false를 반환하고, n이 1이 되면 true를 반환한다.

   Code

   class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<>();
        while (n != 1 && !set.contains(n)) {
            int sum = 0;
            set.add(n);
            while (n != 0) {
                sum += Math.pow(n % 10, 2);
                n /= 10;
            } n = sum;
        } return n == 1;
    }
   }