# AdventJS 2023: Day 11 Challenge

**The solution to** [**challenge #11**](https://adventjs.dev/en/challenges/2023/11) **of** [**AdventJS 2023**](https://adventjs.dev/en)

[**The solution to the** previous challenge](https://alexvalle.dev/adventjs-2023-day-10-challenge)

[**The solution to the next challenge**](https://alexvalle.dev/adventjs-2023-day-11-challenge)

## Challenge Description

In Santa's workshop, the elves love puzzles 🧠. This year, they have created a special one: a challenge to form a Christmas palindrome.

**A palindrome is a word that reads the same forwards and backward.** The elves want to know if it is possible to form a palindrome by making, at most, one exchange of letters.

Create a function `getIndexsForPalindrome` that receives a string and returns:

* If it is already a palindrome, an empty array.
    
* If it is not possible, null.
    
* If a palindrome can be formed with one change, an array with the two positions (indexes) must be swapped to create it.
    

For example:

```javascript
getIndexsForPalindrome('anna') // []
getIndexsForPalindrome('abab') // [0, 1]
getIndexsForPalindrome('abac') // null
getIndexsForPalindrome('aaaaaaaa') // []
getIndexsForPalindrome('aaababa') // [1, 3]
getIndexsForPalindrome('caababa') // null
```

If the palindrome can be formed with different swaps, **always return the first one found.**

## Analysis

In this challenge, our goal is to find a palindrome by swapping two elements of the string and return the indices of those two elements. If a palindrome cannot be found, return an empty array.

### Input

1. Word (`word`): A string with the word to evaluate.
    

### Output

* An array of two elements with the indices to change to find the palindrome.
    
* If it's already a palindrome, return an empty array.
    
* If none of the above conditions are met, return `null`.
    

### Considerations

* If multiple palindromes can be formed, always return the first one found.
    

## Solution

It can be solved by iterating over each element of the string, and within that iteration, iterating again and swapping elements to compare.

### Code

```javascript
/**
 * Returns the indices of characters that need to be reversed in order to make a word a palindrome.
 *
 * @param {string} word - The word to check for palindromic properties.
 * @return {number[] | null} - An array of two indices representing the characters to be reversed, or null if no such indices exist.
 */
function getIndexsForPalindrome(word) {
  // Check if the given word is already a palindrome
  const isPalindrome = (s) => s === s.split("").reverse().join("");

  // If the word is already a palindrome, return an empty array
  if (isPalindrome(word)) {
    return [];
  }

  // Iterate through all possible pairs of indices
  for (let i = 0; i < word.length; i += 1) {
    for (let j = i + 1; j < word.length; j += 1) {
      // Create a new word by swapping the characters at indices i and j
      const newWord =
        word.substring(0, i) +
        word[j] +
        word.substring(i + 1, j) +
        word[i] +
        word.substring(j + 1);

      // If the new word is a palindrome, return the indices i and j
      if (isPalindrome(newWord)) {
        return [i, j];
      }
    }
  }

  // If no such indices exist, return null
  return null;
}
```

### Community Solutions

Solution by [iswilljr](https://github.com/iswilljr/adventjs/blob/master/2023/challenge-11/challenge-11.ts):

```javascript
function getIndexsForPalindrome(word: string) {
  const _letters = [...word]
  const palindrome = word === [..._letters].reverse().join('')

  let initial: number[] | null = [null, []][+palindrome]
  let index = 0
  let aux = 1

  const letters = [_letters, []][+palindrome]
  let auxLetters = letters.slice(1)

  for (const letter of letters) {
    for (const auxLetter of auxLetters) {
      const w = [...letters]
      w[index] = auxLetter
      w[aux] = letter

      const isPalindrome = +(w.join('') === w.reverse().join(''))
      const isInitialNull = +(initial == null)
      const isDifferentIndex = +(index !== aux)

      const values = [initial, initial, initial, [index, aux]]

      initial = values[isInitialNull + isDifferentIndex + isPalindrome]
      aux++
    }

    index++
    aux = 1
    auxLetters = [[], auxLetters][+(initial == null)]
  }

  return initial
}
```

Solution by [Achalogy](https://github.com/Achalogy/advent-js-2023/blob/main/retos/reto-11/main.ts):

```javascript
function getIndexsForPalindrome(word) {
  let res: any = null
  for (const a of Array.from({ length: word.length }).keys()) {
    for (const b of Array.from({ length: word.length }).keys()) {
      let swapped = [...word]
      let aux = word[a]
      swapped[a] = word[b]
      swapped[b] = aux

      let left = swapped.slice(0, Math.floor(word.length / 2)).join("")
      let right = swapped.slice(Math.ceil(word.length / 2)).reverse().join("")

      res = [
        [
          null
          , [
            []
            , [a, b]
          ][+((a + b) > 0)]
        ][+(left == right)]
        , res
      ][+!!res]
    }
  }
  return res
}
```

And that was the challenge for December 11th and its solutions. Give it a like if you enjoyed the challenge or the solution! Do you have an alternative solution? Leave it in the comments!