leetcode202. Happy Number

202. Happy Number

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

/**
* @Author taotao
* @Date 3/6/17
* @Time O(nk) @Space O(n)
* n为循环的数的数量，k为n的数的平均长度
*/
public class Solution {
public boolean isHappy(int n) {
if(n <= 0) {
return false;
}

HashSet<Integer> set = new HashSet<>();
while (set.add(n)) {
int sum = 0;
while (n > 0) {
sum += (n % 10) * (n % 10);
n = n / 10;
}
if (sum == 1) {
return true;
}
n = sum;
}

return false;
}
}