Add Kth Smallest Number in BST

Author: marttp

kotlin/Trees/KthSmallestNumberInBSTIterative.kt

@@ -0,0 +1,38 @@
+import java.util.Stack
+import ds.TreeNode
+
+/*
+    Definition of TreeNode:
+
+    data class TreeNode(
+        var value: Int,
+        var left: TreeNode? = null,
+        var right: TreeNode? = null
+    )
+*/
+
+fun kthSmallestNumberInBSTIterative(root: TreeNode?, k: Int): Int {
+    val stack = Stack<TreeNode>()
+    var node = root
+    // Copy 'k' to a mutable variable so we can decrement it.
+    var k = k
+    while (stack.isNotEmpty() || node != null) {
+        // Move to the leftmost node and add nodes to the stack as we go so they
+        // can be processed in future iterations.
+        while (node != null) {
+            stack.push(node)
+            node = node.left
+        }
+        // Pop the top node from the stack to process it, and decrement 'k'.
+        node = stack.pop()
+        k--
+        // If we have processed 'k' nodes, return the value of the 'k'th smallest
+        // node.
+        if (k == 0) {
+            return node.value
+        }
+        // Move to the right subtree.
+        node = node.right
+    }
+    return -1
+}

kotlin/Trees/KthSmallestNumberInBSTRecursive.kt

@@ -0,0 +1,22 @@
+/*
+    Definition of TreeNode:
+
+    data class TreeNode(
+        var value: Int,
+        var left: TreeNode? = null,
+        var right: TreeNode? = null
+    )
+*/
+
+fun kthSmallestNumberInBSTRecursive(root: TreeNode?, k: Int): Int {
+    val sortedList = inorder(root)
+    return sortedList[k - 1]
+}
+
+// Inorder traversal function to attain a sorted list of nodes from the BST.
+fun inorder(node: TreeNode?): List<Int> {
+    if (node == null) {
+        return emptyList()
+    }
+    return inorder(node.left) + listOf(node.value) + inorder(node.right)
+}