98. Validate Binary Search Tree

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

A valid 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.

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

Constraints:

• The number of nodes in the tree is in the range [1, 104].
• -231 <= Node.val <= 231 - 1

# @lc code=start
using LeetCode

function is_valid_BST(root)
    isnothing(root) && return true
    arr = Int[]
    function pre_tra(root::TreeNode{Int})
        pre_tra(root.left)
        push!(arr, root.val)
        return pre_tra(root.right)
    end
    pre_tra(::Nothing) = nothing
    pre_tra(root)
    return issorted(arr; lt=!isless, rev=true)
end
# @lc code=end

is_valid_BST (generic function with 1 method)

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