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function decodeRailFenceCipher(ciphertext, rails) {
  const zigzag = [];
  for (let i = 0; i < rails; i++) {
    zigzag.push([]);
  }
  let rail = 0;
  let direction = 1;
  for (let i = 0; i < ciphertext.length; i++) {
    zigzag[rail].push(null);
    rail += direction;
    if (rail === rails - 1 || rail === 0) {
      direction = -direction;
    }
  }
  let index = 0;
  for (let i = 0; i < rails; i++) {
    for (let j = 0; j < zigzag[i].length; j++) {
      if (zigzag[i][j] === null) {
        zigzag[i][j] = ciphertext[index++];
      }
    }
  }
  rail = 0;
  direction = 1;
  let plaintext = '';
  for (let i = 0; i < ciphertext.length; i++) {
    plaintext += zigzag[rail][0];
    zigzag[rail].splice(0, 1);
    rail += direction;
    if (rail === rails - 1 || rail === 0) {
      direction = -direction;
    }
  }
  return plaintext;
}