LeetCode第202题. Happy Number

题目:

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: 19 is a happy number

• 12 + 92 = 82
• 82 + 22 = 68
• 62 + 82 = 100
• 12 + 02 + 02 = 1

答案 : 思路其实跟第258题差不多,只不过本题不能单单只通过循环/迭代去解决问题, 而是还要去判断当这个数不是happy number的时 什么时候该停止.

因此,我们需要一个集合,将出现过的数字加入到集合当中,同时判断他本身是否为happy number. 如果不是,则继续迭代去判断.如果再次出现这个数字,则不能加入到集合当中,此时可以判断出他不是一个happy number,

Answer:

 public class Solution {
    public boolean isHappy(int num) {
        Set<Integer> set = new HashSet<Integer>();
        while(num != 1){
            if(set.add(num)==false){
                return false;
            }
            num = getNext(num);
        }
        return true;
    }
    
    int getNext(int num){
        int sum = 0;
        String s = String.valueOf(num);
        for(char c : s.toCharArray()){
            sum += Math.pow(Integer.parseInt(c + ""),2);
        }
        return sum;
    }
}