題目

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: 

Input: 19
Output: true
Explanation: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

十分鐘嘗試

我的嘗試思路，以前處理過int型別資料如何求出每一位，對，就是求mod運算，然後平方相加就可以。程式碼如下：

class Solution {
    public boolean isHappy(int n) {
        
         int res=0;
        while(true){
            while(n>0){
                int mod=n%10;
                res+=Math.pow(mod,2);
                n=n/10;  
            }
            if(res==1){
                return true;
            }
            n=res; 
        }
        return false;
    }
}
 

問題是無限迴圈，leetcode測試不通過。題目也說的是loop endlessly。不知道這個地方如何處理。看了他人對答案，用set.儲存當前的和，如果能add，繼續。另外，上面的程式碼有個問題，res應該在第一個while迴圈裡面，修改如下，完美通過；

class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set=new HashSet();
        while(set.add(n)){
            int res=0;
            while(n>0){
                int mod=n%10;
                res+=Math.pow(mod,2);
                n=n/10;  
            }
            if(res==1){
                return true;
            }
            n=res; 
        }
        return false;
    }
}