Java [Leetcode 202]Happy Number

题目描述：

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

解题思路：

用Set记录平方和的值，如果该值出现过，则表明进入新的循环，返回false；否则继续计算平方和，直到和为1为止。

代码如下：

public class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<Integer>();
        int sum = digitSquareSum(n);
        while(sum != 1){
        	if(!set.add(sum))
        		return false;
        	sum = digitSquareSum(sum);
        }
        return true;
    }

    public int digitSquareSum(int n){
    	int res = 0;
    	while(n > 0){
    		res += (n % 10) * (n % 10);
    		n /= 10;
    	}
    	return res;
    }
}