一、題目

  Given a binary tree, determine if it is a valid binary search tree (BST).

  Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node’s key.
• The right subtree of a node contains only nodes with keys greater than the node’s key.
• Both the left and right subtrees must also be binary search trees.

二、題目大意

  驗證一棵二叉樹是否為二叉搜尋樹。

三、解題思路

  遞迴遍歷出該二叉樹的中序遍歷陣列，判斷該陣列是否為單調遞增陣列。

四、程式碼實現

const isValidBST = root => {
  const ans = []
  help(root)
  for (let i = 0, max = ans.length; i < max - 1; i++) {
    const pre = ans[i]
    const next = ans[i + 1]
    if (next <= pre) {
      return false
    }
  }
  return true
  function help (root) {
    if (!root) {
      return
    }
    help(root.left)
    ans.push(root.val)
    help(root.right)
  }
}
 

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