HappyNumber202.java

/**
 * 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: 
 * 
 * Input: 19
 * Output: true
 * Explanation: 
 * 12 + 92 = 82
 * 82 + 22 = 68
 * 62 + 82 = 100
 * 12 + 02 + 02 = 1
 */

public class HappyNumber202 {
    public boolean isHappy(int n) {
        if (n <= 0) return false;
        Set<Integer> set = new HashSet<>();
        set.add(n);
        while (true) {
            int next = next(n);
            if (next == 1) return true;
            if (set.contains(next)) return false;
            set.add(n);
            n = next;
        }
    }

    private int next(int n) {
        int res = 0;
        while (n != 0) {
            int t = n % 10;
            res += t * t;
            n /= 10;
        }
        return res;
    }


    /**
     * https://leetcode.com/problems/happy-number/discuss/56917/My-solution-in-C(-O(1)-space-and-no-magic-math-property-involved-)
     */
    public boolean isHappy2(int n) {
        int slow, fast;
        slow = fast = n;
        do {
            slow = digitSquareSum(slow);
            fast = digitSquareSum(fast);
            fast = digitSquareSum(fast);
        } while(slow != fast);
        if (slow == 1) return true;
        else return false;
    }

    private int digitSquareSum(int n) {
        int sum = 0, tmp;
        while (n != 0) {
            tmp = n % 10;
            sum += tmp * tmp;
            n /= 10;
        }
        return sum;
    }

}