### 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

Binary tree [2,1,3], return true.

Example 2:

    1
/ \
2   3

Binary tree [1,2,3], return false.

Seen this question in a real interview before?
When did you encounter this question?
Which company?
Send Feedback