202. Happy Number

Posted 2020-10-11 andywu

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

题目含义：对任意一个正整数，求每个数位上数字的平方并加和后得出新的正整数，循环此过程若最终收敛为1，则该数字为happy number，否则程序可能从某个数开始陷入循环。 

 1     public boolean isHappy(int n) {
 2         if (n < 1)  return false;
 3         HashSet<Integer> set = new HashSet<>(32);// 用于保存中间出现的结果
 4         set.add(1);
 5         int tmp;
 6         int newN;
 7         // n的值不能重复出现，否则会死循环
 8         while (!set.contains(n)) {
 9             set.add(n);
10             newN = 0;
11             while (n > 0) {
12                 tmp = n % 10;
13                 n /= 10;
14                 newN += tmp * tmp;
15             }
16             n = newN;
17         }
18         return n == 1;        
19     }