🏂 Beginner-Friendly Guide: "Sum of k-Mirror Numbers" – LeetCode 2081 (python| JavaScript | Python )

Berlin Techs — Fri, 27 Jun 2025 05:13:41 +0000

👋 Introduction
In the world of programming puzzles, palindrome-based problems always offer a unique challenge—especially when they span across multiple number bases. Imagine trying to find numbers that not only look the same forwards and backwards in base-10, but also in base-k. That’s the crux of the k-mirror number challenge.

Let’s dive into the formal problem definition to see how this works:

🧠 Problem Summary
You're given two integers:

k: the base in which we check for palindromes
n: the number of k-mirror numbers to find
A k-mirror number is a positive integer that:

Is a palindrome in base-10
Is also a palindrome in base-k
Your task: return the sum of the first n such numbers.

🧩 Intuition
This is a palindrome-generation and filtering problem. The challenge lies in generating candidate numbers efficiently and checking their representations in different bases.

Key Observations:

Not every decimal palindrome is a k-palindrome (i.e., palindrome in base-k)
Instead of checking all numbers, generate only palindromes in base-10 (which drastically reduces search space)
For each generated number, convert it to base-k and check if that string is also a palindrome
🪄 Approach
Start from the smallest base-10 palindromes: 1-digit numbers.
Use a helper function to generate the next decimal palindrome.
For each candidate:
Convert it to base-k
Check if that base-k representation is a palindrome
If it is, add it to the sum and count
Stop when we find n such numbers
💻 C++ Code
class Solution {
public:
bool isPalindrome(string s) {
return s == string(s.rbegin(), s.rend());
}

string toBaseK(long long num, int k) {
    string res;
    while (num > 0) {
        res += to_string(num % k);
        num /= k;
    }
    reverse(res.begin(), res.end());
    return res;
}

long long kMirror(int k, int n) {
    long long sum = 0;
    int len = 1;
    while (n > 0) {
        for (int half = pow(10, (len - 1) / 2); half < pow(10, (len + 1) / 2) && n > 0; ++half) {
            string h = to_string(half);
            string r = h;
            reverse(r.begin(), r.end());
            string full = h + (len % 2 ? r.substr(1) : r);
            long long num = stoll(full);
            if (isPalindrome(toBaseK(num, k))) {
                sum += num;
                --n;
            }
        }
        ++len;
    }
    return sum;
}

};
💻 JavaScript Code
function isPalindrome(s) {
return s === s.split('').reverse().join('');
}

function toBaseK(num, k) {
return num.toString(k);
}

    def to_base_k(num, k):
        res = ""
        while num:
            res = str(num % k) + res
            num //= k
        return res

    def generate_palindromes():
        length = 1
        while True:
            for half in range(10 ** ((length - 1) // 2), 10 ** ((length + 1) // 2)):
                s = str(half)
                yield int(s + s[-2::-1] if length % 2 else s + s[::-1])
            length += 1

    ans = 0
    count = 0
    for num in generate_palindromes():
        if is_palindrome(to_base_k(num, k)):
            ans += num
            count += 1
            if count == n:
                break
    return ans

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🏂 Beginner-Friendly Guide: "Sum of k-Mirror Numbers" – LeetCode 2081 (python| JavaScript | Python )