202. Happy Number

Posted 2021-01-20 Veritas des Liberty

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

 

Approach #1: C++.

class Solution {
public:
    bool isHappy(int n) {
        unordered_set<int> s;
        s.insert(n);
        while (1) {
            int summ = 0;
            while (n) {
                int t = n % 10;
                summ += t*t;
                n /= 10;
            }
            if (summ == 1) return true;
            if (s.count(summ)) return false;
            s.insert(summ);
            n = summ;
        }
    }
};

　　

Approach #2: Java.

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

　　

Approach #3: Python.

class Solution(object):
    def isHappy(self, n):
        """
        :type n: int
        :rtype: bool
        """
        mem = set()
        while n != 1:
            n = sum([int(i)**2 for i in str(n)])
            if n in mem:
                return False
            else:
                mem.add(n)
        else:
            return True

　　

Time Submitted	Status	Runtime	Language
a few seconds ago	Accepted	28 ms	python
2 minutes ago	Accepted	5 ms	java
8 minutes ago	Accepted	0 ms	cpp