LeetCode -- Validate Binary Search Tree

LeetCode -- Validate Binary Search Tree
Description:


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.


Is to determine whether a binary tree is a search binary tree.


The search binary tree is defined as: each node in the left subtree is smaller than the current node; each node in the right subtree is greater than the current node.


Idea: traverse nodes in the middle order and store them in an array, and then judge whether the array is incremented (not equal ).


Implementation Code:

/** * Definition for a binary tree node. * public class TreeNode { *     public int val; *     public TreeNode left; *     public TreeNode right; *     public TreeNode(int x) { val = x; } * } */public class Solution {    public bool IsValidBST(TreeNode root)     {        var result = new List
 
  ();    Travel(root, ref result);        for(var i = 0;i < result.Count - 1; i++){    if(result[i+1] <= result[i]){    return false;    }    }        return true;    }private void Travel(TreeNode node ,ref List
  
    result){if(node == null){return;}Travel(node.left, ref result);result.Add(node.val);Travel(node.right, ref result);}}