leetcode之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

这道题很简单，思路大致如下，首先取这个数的每一位数，其次考虑到终止条件是1或者循环

最开始只判断了最终条件如果是1就结束的情况，忘记了如果无限循环会怎么样，于是没有AC。

从网上查找了一下资料，发现用HashSet 是比较合理的，首先add()方法会将不在集合中的元素加入，其次contain()方法会判断某个数是否在集合内，

因此写了两个函数，一个是得到下一个要判断的数，另一个是判断这个数是不是Happy Number

代码如下：

public class Solution {
    public boolean isHappy(int n) {
        HashSet<Integer> hash = new HashSet<Integer>();
        while (n != 1) {
            if (hash.contains(n)) {
                return false;
            }
            hash.add(n);
            n = s(n);
        }
        return true;
    }
    public int s(int n){
		int num = 0;
        while(n!=0){
        	int u = n/10;
        	int v = n%10;
        	n = u;
        	num += v*v;
        }
        int result = num;
        return result;
	}
}