Add Build Binary Tree

Author: marttp

kotlin/Trees/BuildBinaryTree.kt

@@ -0,0 +1,65 @@
+import ds.TreeNode
+
+/*
+    Definition of TreeNode:
+
+    data class TreeNode(
+        var value: Int,
+        var left: TreeNode? = null,
+        var right: TreeNode? = null
+    )
+*/
+
+fun buildBinaryTree(preorder: List<Int>, inorder: List<Int>): TreeNode? {
+    val inorderIndexesMap = hashMapOf<Int, Int>()
+    val preorderIndex = intArrayOf(0)
+    // Populate the hash map with the inorder values and their indexes.
+    for (i in inorder.indices) {
+        inorderIndexesMap[inorder[i]] = i
+    }
+    // Build the tree and return its root node.
+    return buildSubtree(0, inorder.size - 1, preorder, inorder, preorderIndex, inorderIndexesMap)
+}
+
+fun buildSubtree(
+        left: Int,
+        right: Int,
+        preorder: List<Int>,
+        inorder: List<Int>,
+        preorderIndex: IntArray,
+        inorderIndexesMap: Map<Int, Int>
+): TreeNode? {
+    // Base case: if no elements are in this range, return null.
+    if (left > right) {
+        return null
+    }
+    val value = preorder[preorderIndex[0]]
+    // Set 'inorderIndex' to the index of the same value pointed at by
+    // 'preorderIndex'.
+    val inorderIndex = inorderIndexesMap[value]!!
+    val node = TreeNode(value)
+    // Advance 'preorderIndex' so it points to the value of the next
+    // node to be created.
+    preorderIndex[0]++
+    // Build the left and right subtrees and connect them to the current
+    // node.
+    node.left =
+            buildSubtree(
+                    left,
+                    inorderIndex - 1,
+                    preorder,
+                    inorder,
+                    preorderIndex + 1,
+                    inorderIndexesMap
+            )
+    node.right =
+            buildSubtree(
+                    inorderIndex + 1,
+                    right,
+                    preorder,
+                    inorder,
+                    preorderIndex + inorderIndex - left + 1,
+                    inorderIndexesMap
+            )
+    return node
+}