Question

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.

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

Thinking:

  • Method:
    • 使用递归，将不符合的所有数字都加入集合中。
    • 如果碰到了集合中的数字则返回false。
    • 如果碰到了正确的就返回true。
class Solution {
    public boolean isHappy(int n) {
        Set<Integer> s = new HashSet<>();
        return isHappy(n, s);
    }
    private boolean isHappy(int n, Set<Integer> s){
        if(s.contains(n)) return false;
        else{
            s.add(n);
            int cur = 0;
            int remain = 0;
            int sum = 0;
            while(n != 0){
                remain = n % 10;
                n /= 10;
                sum += Math.pow(remain, 2);
            }
            if(sum == 1) return true;
            else{
                return isHappy(sum, s);
            }
        }
    }
}

二刷

  1. 二刷使用了循环，还是要用一个set去存储已经访问的数字。 
    class Solution {
 public boolean isHappy(int n) {
     if(n <= 0) return false;
     Set<Integer> visited = new HashSet<>();
     List<Integer> temp = null;
     while(!visited.contains(n)){
         visited.add(n);
         temp = new ArrayList<>();
         getDigits(temp, n);
         int count = 0;
         for(Integer d : temp){
             count += d * d;
         }
         if(count == 1) return true;
         n = count;
     }
     return false;
 }
    
 private void getDigits(List<Integer> list, int n){
     while(n != 0){
         list.add(n % 10);
         n /= 10;
     }
 }
}