Link: https://leetcode.cn/problems/happy-number/

Question

difficulty: easy
adj diff: 3

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

注意避免数字循环（用记忆性搜索）。

Code

public boolean isHappy(int n) {
    return isHappy(new HashSet<Integer>(), n);
}

private boolean isHappy(Set<Integer> set, int n) {
    if (n == 1) return true;
    if (set.contains(n)) return false;
    set.add(n);
    int sum = 0;
    while (n != 0) {
        sum += (n % 10) * (n % 10);
        n /= 10;
    }
    return isHappy(set, sum);
}