Add Binary Search Tree Validation

Author: marttp

kotlin/Trees/BinarySearchTreeValidation.kt

@@ -0,0 +1,36 @@
+import ds.TreeNode
+
+/*
+    Definition of TreeNode:
+
+    data class TreeNode(
+        var value: Int,
+        var left: TreeNode? = null,
+        var right: TreeNode? = null
+    )
+*/
+
+fun binarySearchTreeValidation(root: TreeNode?): Boolean {
+    // Start validation at the root node. The root node can contain any
+    // value, so set the initial lower and upper bounds to -infinity and
+    // +infinity, respectively.
+    return isWithinBounds(root, Int.MIN_VALUE, Int.MAX_VALUE)
+}
+
+fun isWithinBounds(node: TreeNode?, lowerBound: Int, upperBound: Int): Boolean {
+    // Base case: if the node is null, it satisfies the BST condition.
+    if (node == null) {
+        return true
+    }
+    // If the current node's value is not within the valid bounds, this
+    // tree is not a valid BST.
+    if (node.value <= lowerBound || node.value >= upperBound) {
+        return false
+    }
+    // If the left subtree isn't a BST, this tree isn't a BST.
+    if (!isWithinBounds(node.left, lowerBound, node.value)) {
+        return false
+    }
+    // Otherwise, return true if the right subtree is also a BST.
+    return isWithinBounds(node.right, node.value, upperBound)
+}