Update

105_Construct_Binary_Tree_from_Preorder_and_Inorder_Traversal.java

Author: Sean Prashad

Tree Depth First Search/105_Construct_Binary_Tree_from_Preorder_and_Inorder_Traversal.java

@@ -1,30 +1,26 @@
 class Solution {
     public TreeNode buildTree(int[] preorder, int[] inorder) {
-        if (preorder == null || inorder == null || preorder.length != inorder.length) {
-            return null;
-        }
-
-        return buildTreeHelper(preorder, inorder, 0, 0, inorder.length - 1);
+        return helper(preorder, inorder, 0, preorder.length - 1, 0, inorder.length - 1);
     }
 
-    private TreeNode buildTreeHelper(int[] preorder, int[] inorder, int preStart, int inStart, int inEnd) {
-        if (preStart > preorder.length - 1 || inStart > inEnd) {
+    private TreeNode helper(int[] preorder, int[] inorder, int preStart, int preEnd, int inStart, int inEnd) {
+        if (preStart > preEnd || inStart > inEnd) {
             return null;
         }
 
         TreeNode root = new TreeNode(preorder[preStart]);
+        int rootIdx = 0;
 
-        int inOrderIdx = preStart;
-        for (int i = 0; i < inorder.length; i++) {
-            if (root.val == inorder[i]) {
-                inOrderIdx = i;
+        for (int i = inStart; i <= inEnd; i++) {
+            if (inorder[i] == root.val) {
+                rootIdx = i;
                 break;
             }
         }
 
-        int leftTreeLength = inOrderIdx - inStart;
+        root.left = helper(preorder, inorder, preStart + 1, preEnd, inStart, rootIdx - 1);
+        root.right = helper(preorder, inorder, preStart + rootIdx - inStart + 1, preEnd, rootIdx + 1, inEnd);
 
-        root.left = buildTreeHelper(preorder, inorder, preStart + 1, inStart, inOrderIdx - 1);
-        root.right = buildTreeHelper(preorder, inorder, preStart + leftTreeLength + 1, inOrderIdx + 1, inEnd);
+        return root;
     }
 }