◻️202. Happy Number (easy)

Write an algorithm to determine if a number n 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.

• 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.

Return true if n is a happy number, and false if not.

Example 1:

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

Example 2:

Input: n = 2
Output: false

---

Solutions

Normal - Linked List 2 Pointers

Runtime 7 ms Memory 7.45 MB

class Solution {
public:
    bool isHappy(int n) {
        int slow=n, fast=n;
        slow = square_fn(slow);
        fast = square_fn(square_fn(fast));
        cout << "slow: " << slow << " fast: " << fast << endl;
        while(slow != fast){
            slow = square_fn(slow);
            fast = square_fn(square_fn(fast));
            cout << "slow: " << slow << " fast: " << fast << endl;
            if(slow == fast){
                if(slow == 1) return(true);
                else return(false);
            }
        }
        return(true);
    }
private:
    int square_fn(int n){
        int sum = 0;
        while(n != 0){
            sum += pow(n%10, 2);
            n = n/10;
        }
        return sum;
    }
};

Optimised - HashSet

Runtime 5 ms Memory 8.22 MB

T.C -> O(1) | S.C -> O(1)

```cpp
class Solution {
public:
    bool isHappy(int n) {
        unordered_set<int> mpp;
        while(true){
            int sum=0;
            while(n != 0){
                sum += pow(n%10, 2);    // Gives last digit
                n=n/10;                 // Removes last digit
            }
            if(sum == 1) return(true);
            if(mpp.find(sum) != mpp.end()){
                return false;
            }
            else{  
                mpp.insert(sum);
                n=sum;                  // Important line
            }
        }
        return false;
    }
};
```