Happy Number

Write an algorithm to determine if a number 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,
and 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 numbers.

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Example
19 is a happy number

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

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Hash Table Mathematics

思路

• 考察的就是求位数上的数字
• 用hashset保存之前生成的值

public class Solution {
    /**
     * @param n an integer
     * @return true if this is a happy number or false
     */
    public boolean isHappy(int n) {
        // Write your code here
        if (n <= 0) {
            return false;
        }

        int sum = 0;
        HashSet<Integer> set = new HashSet<Integer>();
        set.add(n);

        while (true) {
            while (n > 0) {
                sum = (n % 10) * (n % 10) + sum;
                n /= 10;
            }
            n = sum;
            sum = 0;

            if (n == 1) {
                return true;
            }
            if (set.contains(n)) {
                break;
            }

            set.add(n);
        }

        return false;
    }
}