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.
2 / \ 1 3 Binary tree [2,1,3], return true.
1 / \ 2 3 Binary tree [1,2,3], return false.
Please check the main.js snippet for the solution.
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