202. Happy Number

• Difficulty: Easy

• Topics: Hash Table, Math

• Similar Questions:

  • Linked List Cycle

  • Add Digits

  • Ugly Number

Problem:

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

Solutions:

class Solution {
public:
    bool isHappy(int n) {

        if (n <= 0) return false;
        if (n == 1) return true;
        unordered_set<int> seen;

        int nextVal = 0;
        for (;;) {
            nextVal = 0; // reset nextVal
            while (n > 0) {
                nextVal += (n%10) * (n%10);
                n /= 10;
            }

            if (nextVal == 1) return true;
            if (seen.count(nextVal) > 0)    return false;
            seen.insert(nextVal);
            n = nextVal;
        }

    }
};