98. Validate Binary Search Tree

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 is binary search trees.

Idea: The middle order is arranged in a sorted array (the array element is incremented at a time), and the array cannot have duplicate elements.

The code is as follows:

1 /**2 * Definition for a binary tree node.3 * public class TreeNode {4 * int val;5 * TreeNode left;6 * TreeNode right;7 * TreeNode (int x) {val = x;}8  * }9  */Ten  Public classSolution { One      Public BooleanIsvalidbst (TreeNode root) { A        -         if(root==NULL|| (root.left==NULL&&root.right==NULL)) -         return true; theArraylist<integer> list=Parentandson (root); -           int[] nums=New int[List.size ()]; -           int[] nums1=New int[List.size ()]; -            for(intI=0;i<list.size (); i++) +           { -nums[i]=List.get (i); +nums1[i]=List.get (i); A           } at Arrays.sort (nums); -      -           if(nums[0]!=nums1[0]) -           return false; -            for(inti=1;i<nums.length;i++) -           { in            if(nums1[i]!=nums[i]| | Nums[i]==nums[i-1]) -            return false; to            +           } -      return true; the     } *      PublicArraylist<integer>Parentandson (TreeNode root) { $Arraylist<integer> list=NewArraylist<>();Panax Notoginseng         if(root.left!=NULL) - List.addall (Parentandson (Root.left)); the          + List.add (root.val); A          the         if(root.right!=NULL) + List.addall (Parentandson (root.right)); -         returnlist; $     } $}

98. Validate Binary Search Tree