Happy Number
題意：
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
$$1^2 + 9^2 = 82$$
$$8^2 + 2^2 = 68$$
$$6^2 + 8^2 = 100$$
$$1^2 + 0^2 + 0^2 = 1$$
解題思路：
一個數是happy number只以下兩個條件
- 數有循環，且數中不包含1
- 或是數最後走到1
public class Solution {
public boolean isHappy(int n) {
HashSet<Integer> set = new HashSet<Integer>();
while (n != 1 && !set.contains(n)) {
set.add(n);
int sum = 0;
while (n != 0) {
sum += Math.pow(n % 10, 2);
n /= 10;
}
n = sum;
}
return n == 1;
}
}