一、題目

  One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node’s value. If it is a null node, we record using a sentinel value such as #.

       _9_
      /   \
     3     2
    / \   / \
   4   1  #  6
  / \ / \   / \
  # # # #   # #
 

  For example, the above binary tree can be serialized to the string “9,3,4,#,#,1,#,#,2,#,6,#,#”, where # represents a null node.

  Given a string of comma separated values, verify whether it is a correct preorder traversal serialization of a binary tree. Find an algorithm without reconstructing the tree.

  Each comma separated value in the string must be either an integer or a character ‘#’ representing null pointer.

  You may assume that the input format is always valid, for example it could never contain two consecutive commas such as “1,3”.

二、題目大意

  給定二叉樹的前序遍歷字串，驗證該字串是否有效，（空節點採用’#'表示）。

三、解題思路

  這道題參考了社群大神的解法，簡單的理解就是：在遍歷的過程中，如果系統的入度大於出度，那麼“空閒分支”已經用完，後續節點無法掛載在二叉樹上，從而判斷字串是否有效。

四、程式碼實現

const isValidSerialization = preorder => {
  const a = preorder.split(',')
  let diff = 1
  for (let item of a) {
    diff -= 1
    if (diff < 0) {
      return false
    }
    if (item !== '#') {
      diff += 2
    } 
  }
  return diff === 0
}

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