202. Happy Number

题目：

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

链接： http://leetcode.com/problemset/algorithms/

2/23/2017, Java

注意HashSet的API, performance并不好

 1 public class Solution {
 2     public boolean isHappy(int n) {
 3         if (n == 0) return false;
 4         HashSet<Integer> s = new HashSet<Integer>();
 5         int val = 0, tmp;
 6 
 7         while (val != 1 && !s.contains(val)) {
 8             s.add(val);
 9             val = 0;
10             while (n > 0) {
11                 tmp = n % 10;
12                 val += tmp * tmp;
13                 n /= 10;
14             }
15             n = val;
16         }
17 
18         if (val == 1) return true;
19         return false;
20         
21     }
22 }

别人的算法

更好的解可以把Space Complexity 简化到 O(1)，使用 fast / slow pointer进行Cycle Detection的思路，很巧妙。更奇妙的是运行时间也减少了。

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