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 be binary search trees.

Example 1:

``````2
``````

/ \
1 3

Input: [2,1,3]
Output: true
Example 2:

``````5
``````

/ \
1 4
/ \
3 6

Input: [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.

O(N) O(1)

Code

``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {
private TreeNode pre;
private boolean res = true;
public boolean isValidBST(TreeNode root) {
helper(root);
return res;
}
public void helper(TreeNode root){
if(root == null)
return;
helper(root.left);

if(pre != null && pre.val >= root.val)
res = false;
pre = root;

helper(root.right);
}
}
``````