*Happy Number

標籤：

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

• 12 + 92 = 82
• 82 + 22 = 68
• 62 + 82 = 100
• 12 + 02 + 02 = 1

 

Naive Way: The statement is describing a recursive process. So it is obvious that the question want me to write a recursive method. The logic has been stated and clear. Base case (ending condition) is either n=1 or n has been visited. And since I need to mark whether a number has been visited, a hashset is required.

 

 

 1  public class Solution {   2    Set<Integer> visited = new HashSet<Integer>();    這個要定義在前面 3    public boolean isHappy(int n) {   4      // base case   5      if(n==1) return true;   6      // visited   7      if(visited.contains(n)) return false;   8      // main processing   9      int sum = 0;  10      visited.add(n);  11      while(n!=0){  12        int lastDigit = n % 10;  13        sum += lastDigit * lastDigit;  14        n /= 10;  15      }  16      return isHappy(sum);  17    }  18  }  

reference: http://siyang2leetcode.blogspot.com/2015/05/happy-number.html

*Happy Number