Happy Number

Problem Statement

You’re a number mystic checking if a number is "happy"—repeatedly sum the squares of its digits until it reaches 1 (happy) or loops (unhappy). This easy-level set challenge is a cyclic quest—use hashing to spot the loop!

Example

Input: n = 19

Output: true (1²+9²=82, 8²+2²=68, ..., 1)

Input: n = 2

Output: false (Loops: 4, 16, 37, ...)

Input: n = 7

Output: true (Reaches 1)

Code

public class Solution {
    public boolean isHappy(int n) {
        Set seen = new HashSet<>();
        while (n != 1 && !seen.contains(n)) {
            seen.add(n);
            n = getSumOfSquares(n);
        }
        return n == 1;
    }
    private int getSumOfSquares(int n) {
        int sum = 0;
        while (n > 0) {
            int digit = n % 10;
            sum += digit * digit;
            n /= 10;
        }
        return sum;
    }
}
            

def is_happy(n):
    seen = set()
    while n != 1 and n not in seen:
        seen.add(n)
        n = sum(int(d) ** 2 for d in str(n))
    return n == 1
            

function isHappy(n) {
    let seen = new Set();
    while (n !== 1 && !seen.has(n)) {
        seen.add(n);
        n = String(n).split('').reduce((sum, d) => sum + d * d, 0);
    }
    return n === 1;
}
            

Explanation

• Set Insight: Detect cycles with a set of seen numbers.
• Flow: Compute sum of squares, check if 1 or looped, repeat.
• Example Walkthrough: 19 → 82 → 68 → 65 → 61 → 37 → 58 → 89 → 145 → 42 → 20 → 4 → 16 → 37 (loop) → false.
• Math Fact: Unhappy numbers enter a cycle (e.g., 4→16→37).
• Alternative: Floyd’s cycle detection works too.

Note