Question:

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.

Thinking:

We should use dfs check the root of the range and both its left child and right child.

Solution:

public boolean isValidBST(TreeNode root) {
    return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
}

private boolean helper(TreeNode root, long a, long b) {
    if (root == null)    return true;
    if (root.val <= a || root.val >= b)    return false;        
    return helper(root.left, a, root.val<b? root.val:b) && helper(root.right, root.val>a? root.val:a, b);
}