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# 0202. Happy Number

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

## Description

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`

## ac

```java
class Solution {
    public boolean isHappy(int n) {
        // edge cases
        if (n <= 0) return false;

        return recur(n, new HashSet<Integer>());
    }

    private boolean recur(int n, Set<Integer> set) {
        // exit
        if (n == 1) return true;
        if (set.contains(n)) return false;  // meet loop

        set.add(n);
        int m = 0;
        while (n > 0) {
            m += Math.pow(n % 10, 2);
            n /= 10;
        }

        // next
        return recur(m, set);
    }
}
```