LeetCode 202. Happy Number （快乐数字）

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

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 题目标签：Hash Table

　　题目给了我们一个数字，让我们判断它是不是 Happy Number。

　　Happy Number 最终会变成1，很好判断；Non-Happy Number 最后会无限循环在一个cycle。

　　所以，我们要解决如何判断 一个数字是否在一个无限循环里，可以建立HashSet 记录它每一次的变化数字，一旦它重复了，说明，它在循环。

Java Solution:

Runtime beats 61.14% 

完成日期：05/16/2017

关键词：HashSet

关键点：利用HashSet 记录每一个数字

class Solution 
{
    public boolean isHappy(int n) 
    {
        HashSet<Integer> set = new HashSet<>();
        
        while(n != 1) // happy number will get out of this loop
        {
            if(set.contains(n)) // if a number repeats in a cycle, return false;
                return false;
            
            set.add(n);
            
            int sum = 0;
            
            while(n != 0) // get each digit from number
            {
                int digit = n % 10;
                
                sum += digit * digit;
                n = n / 10;
            }
            
            n = sum; // update n with new number
        }
        
        return true;
    }
}

参考资料：N/A

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