LeetCode:Happy Number

问题描述：

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

思路：

设置一个set存储已经出现过的数字，防止出现死循环，处理数字的函数按照规则进行，将数字的每个位置都进行平方而后相加即可。

java代码：

public class Solution {
    public boolean isHappy(int n) {
        if(n < 1)  return false;
        if(n == 1) return true;
        Set<Long> set = new HashSet<Long>();
        long ln = n;
        while(ln <= Integer.MAX_VALUE){
            if(set.contains(ln)) return false;
            else set.add(ln);
            ln = digitSquare(ln);
            if(ln == 1)     return true;
        }
        return false;
    }
    
    private long digitSquare(long ln) {  
        long sum = 0;  
        while(ln != 0) {  
            sum += Math.pow(ln % 10, 2);  
            ln /= 10;  
        }  
        return sum;  
    }  
}