1
/**
2
 * @param {string} preorder
3
 * @return {boolean}
4
 */
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var isValidSerialization = function (preorder) {
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  let balance = 1;
7
  for (const node of preorder.split(","))
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    if (balance > 0)
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      if (node === "#") --balance;
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      else ++balance;
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    else return false;
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  return balance < 1;
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};