Recover secret string from random triplets

recover-secret.js

// There is a secret string which is unknown to you. Given a collection of 
// random triplets from the string, recover the original string.
// 
// A triplet here is defined as a sequence of three letters such that each 
// letter occurs somewhere before the next in the given string. "whi" is a 
// triplet for the string "whatisup".
// 
// As a simplification, you may assume that no letter occurs more than once in 
// the secret string.
// 
// You can assume nothing about the triplets given to you other than that they 
// are valid triplets and that they contain sufficient information to deduce the 
// original string. In particular, this means that the secret string will never 
// contain letters that do not occur in one of the triplets given to you.
// 
// Test case:
// 
// var secret1 = "whatisup"
// var triplets1 = [
//   ['t','u','p'],
//   ['w','h','i'],
//   ['t','s','u'],
//   ['a','t','s'],
//   ['h','a','p'],
//   ['t','i','s'],
//   ['w','h','s']
// ]
// 
// Test.assertEquals(recoverSecret(triplets1), secret1)




// -----------------------------------------------------------------------------
// SOLUTION
// -----------------------------------------------------------------------------
const recoverSecret = (triplets) => {
  let duplets = [];
  let res = '';

  /*
    convert triplets to duplets
   */
  triplets.forEach(trip => {
    let str = trip.join('');
    let d1 = str.substr(0,2);
    let d2 = str.substr(1,3);
    if (duplets.indexOf(d1) === -1) { duplets.push(d1); } 
    if (duplets.indexOf(d2) === -1) { duplets.push(d2); }
  });
  
  let nxt = findNext(duplets);
  while (nxt) {
    res += nxt;
    duplets = duplets.filter(d => d.indexOf(nxt) === -1);
    nxt = findNext(duplets);
  }
  
  return res;
}

/**
 * Return the first letter of the secret word.
 */
function findNext(duplets) {
  // 1st char is not the 2nd char in any duplet
  let nxt = duplets.find(d => duplets.every(_d => d[0] !== _d[1])); 
  return duplets.length > 1 ? nxt[0] : duplets[0];
}