Add binary tree from preorder and inorder traversal

Author: sudheerj

src/java1/algorithms/tree/BuildTree.java

@@ -0,0 +1,30 @@
+package java1.algorithms.tree;
+
+import java.util.Arrays;
+
+public class BuildTree {
+    //TC: O(n) SC: O(n)
+    private TreeNode buildTree(int[] preorder, int[] inorder){
+        if(preorder.length == 0 || inorder.length == 0) {
+            return null;
+        }
+
+        TreeNode root = new TreeNode(preorder[0]);
+        int mid = 0;
+        for(int i=0; i<preorder.length; i++) {
+            if(inorder[i] == preorder[0]) {
+                mid = i;
+            }
+        }
+
+        root.left = buildTree(Arrays.copyOfRange(preorder, 1, mid+1), Arrays.copyOfRange(inorder, 0, mid));
+        root.left = buildTree(Arrays.copyOfRange(preorder, mid+1, preorder.length), Arrays.copyOfRange(inorder, mid+1, inorder.length));
+        
+        return root;
+    }
+    public static void main(String[] args) {
+        int[] preorder = {3,9,20,15,7}, inorder = {9,3,15,20,7};
+        BuildTree tree = new BuildTree();
+        System.out.println(tree.buildTree(preorder, inorder));
+    }
+}

src/javascript/algorithms/tree/buildTree.js

@@ -0,0 +1,24 @@
+//TC: O(n) SC: O(n)
+
+function buildTree(preorder, inorder) {
+    if(preorder.length === 0 || inorder.length === 0) return null;
+
+    let root = new TreeNode(preorder[0]);
+    let mid = inorder.indexOf(preorder[0]);
+
+    root.left = buildTree(preorder.slice(1, mid+1), inorder.slice(0, mid));
+    root.right = buildTree(preorder.slice(mid+1, preorder.length), inorder.slice(mid+1, inorder.length));
+
+    return root;
+}
+
+class TreeNode {
+    constructor(value) {
+      this.left = null;
+      this.right = null;
+      this.value = value;
+    }
+}
+
+let preorder = [3,9,20,15,7], inorder = [9,3,15,20,7];
+console.log(buildTree(preorder, inorder));