freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-javascript/69f35a5bb823ed620fcb7cb8.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
69f35a5bb823ed620fcb7cb8	Challenge 279: Longest Domino Chain	28	challenge-279

--description--

Given a 2D array representing a set of dominoes, return the longest valid chain.

• Each domino is a pair of numbers from 0–6, e.g. [3, 2].
• A chain is valid when the second number of each domino matches the first number of the next.
• The first number of the first domino and the second number of the last one don't need to match anything.
• Any domino can be flipped, so [3, 2] can be played as [2, 3].
• There is always exactly one longest valid chain.

For example, given [[1, 2], [4, 5], [2, 3]], return [[1, 2], [2, 3]].

--hints--

getLongestChain([[1, 2], [4, 5], [2, 3]]) should return [[1, 2], [2, 3]].

const chain = JSON.stringify(getLongestChain([[1, 2], [4, 5], [2, 3]]));
const result1 = JSON.stringify([[1, 2], [2, 3]]);
const result2 = JSON.stringify([[3, 2], [2, 1]]);
assert.isTrue(chain === result1 || chain === result2);

getLongestChain([[2, 1], [4, 3], [5, 3]]) should return [[4, 3], [3, 5]].

const chain = JSON.stringify(getLongestChain([[2, 1], [4, 3], [5, 3]]));
const result1 = JSON.stringify([[4, 3], [3, 5]]);
const result2 = JSON.stringify([[5, 3], [3, 4]]);
assert.isTrue(chain === result1 || chain === result2);

getLongestChain([[1, 2], [3, 4], [2, 3], [4, 0]]) should return [[1, 2], [2, 3], [3, 4], [4, 0]].

const chain = JSON.stringify(getLongestChain([[1, 2], [3, 4], [2, 3], [4, 0]]));
const result1 = JSON.stringify([[1, 2], [2, 3], [3, 4], [4, 0]]);
const result2 = JSON.stringify([[0, 4], [4, 3], [3, 2], [2, 1]]);
assert.isTrue(chain === result1 || chain === result2);

getLongestChain([[6, 6], [6, 1], [1, 1], [0, 3], [2, 3], [4, 1], [5, 6]]) should return [[4, 1], [1, 1], [1, 6], [6, 6], [6, 5]].

const chain = JSON.stringify(getLongestChain([[6, 6], [6, 1], [1, 1], [0, 3], [2, 3], [4, 1], [5, 6]]));
const result1 = JSON.stringify([[4, 1], [1, 1], [1, 6], [6, 6], [6, 5]]);
const result2 = JSON.stringify([[5, 6], [6, 6], [6, 1], [1, 1], [1, 4]]);
assert.isTrue(chain === result1 || chain === result2);

getLongestChain([[0, 4], [3, 3], [0, 3], [5, 6], [4, 5], [4, 2], [5, 5], [1, 2], [4, 4]]) should return [[3, 3], [3, 0], [0, 4], [4, 4], [4, 5], [5, 5], [5, 6]].

const chain = JSON.stringify(getLongestChain([[0, 4], [3, 3], [0, 3], [5, 6], [4, 5], [4, 2], [5, 5], [1, 2], [4, 4]]));
const result1 = JSON.stringify([[3, 3], [3, 0], [0, 4], [4, 4], [4, 5], [5, 5], [5, 6]]);
const result2 = JSON.stringify([[6, 5], [5, 5], [5, 4], [4, 4], [4, 0], [0, 3], [3, 3]]);
assert.isTrue(chain === result1 || chain === result2);

--seed--

--seed-contents--

function getLongestChain(dominoes) {

  return dominoes;
}

--solutions--

function getLongestChain(dominoes) {
  function search(chain, remaining) {
    let best = chain;
    const last = chain[chain.length - 1][1];

    for (let i = 0; i < remaining.length; i++) {
      const [a, b] = remaining[i];
      const rest = remaining.filter((_, j) => j !== i);

      if (a === last) {
        const result = search([...chain, [a, b]], rest);
        if (result.length > best.length) best = result;
      }
      if (b === last && a !== b) {
        const result = search([...chain, [b, a]], rest);
        if (result.length > best.length) best = result;
      }
    }
    return best;
  }

  let best = [];
  for (let i = 0; i < dominoes.length; i++) {
    const [a, b] = dominoes[i];
    const rest = dominoes.filter((_, j) => j !== i);

    const r1 = search([[a, b]], rest);
    if (r1.length > best.length) best = r1;

    if (a !== b) {
      const r2 = search([[b, a]], rest);
      if (r2.length > best.length) best = r2;
    }
  }
  return best;
}