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;
 }

}