*Happy Number

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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>();    这个要定义在前面
 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

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原文地址：http://www.cnblogs.com/hygeia/p/4799835.html