202.Happy Number

Tags: [math], [simulation], [digit], [set]

Com: {t}

Hard: [##]

Link: https://leetcode.com/problems/happy-number/#/description

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.

Example: 19 is a happy number

Solution: Simulation, Set

public class Solution {
    public boolean isHappy(int n) {
        if (n <= 0) {
            return false;
        }

        Set<Integer> set = new HashSet<>();
        while (!set.contains(n)) {
            set.add(n);

            if (n == 1) {
                return true;
            }

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

        return false;
    }
}

Revelation:

• Use set to check whether there is an endless partner. There is another way to detect the cycle, by using slow and fast, reference: https://discuss.leetcode.com/topic/12587/my-solution-in-c-o-1-space-and-no-magic-math-property-involved
• In Java, Math.pow(..., ...) return double type.

Note:

• Time complexity depends on the input.