[swift] LeetCode 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.

func isValidBST(_ root: TreeNode?) -> Bool {
    return isValidNode(root, Int.min, Int.max)
}

private func isValidNode(_ root: TreeNode?, _ min: Int, _ max: Int) -> Bool {
    guard let p = root else { return true }
    if p.val >= max || p.val <= min { return false }
    return isValidNode(p.left, min, p.val) && isValidNode(p.right, p.val, max)
}

func isValidBST(_ root: TreeNode?) -> Bool {

return isValidNode(root, Int.min, Int.max)

}

private func isValidNode(_ root: TreeNode?, _ min: Int, _ max: Int) -> Bool {

guard let p = root else { return true }

if p.val >= max || p.val <= min { return false }

return isValidNode(p.left, min, p.val) && isValidNode(p.right, p.val, max)

}

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