solves #106 in java

Author: anishLearnsToCode

Diff for: README.md

@@ -98,7 +98,7 @@
 | 103  | [Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal)                                        |                                                       [![Java](assets/java.png)](src/BinaryTreeZigzagLevelOrderTraversal.java)                                                       |                                                                                                                                   |
 | 104  | [Maximum Depth of Binary Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree)                                                                |                         [![Java](assets/java.png)](src/MaximumDepthOfBinaryTree.java) [![Python](assets/python.png)](python/maximum_depth_of_binary_tree.py)                         |                                                                                                                                   |
 | 105  | [Construct Binary Tree from Preorder and Inorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal)      |                                               [![Java](assets/java.png)](src/ConstructBinaryTreeFromPreorderAndInorderTraversal.java)                                                |                                                                                                                                   |
-| 106  | [Construct Binary Tree from Inorder and Postorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal)    |                                                                                                                                                                                      |                                                                                                                                   |
+| 106  | [Construct Binary Tree from Inorder and Postorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal)    |                                               [![Java](assets/java.png)](src/ConstructBinaryTreeFromInorderAndPostorderTraversal.java)                                               |                                                                                                                                   |
 | 107  | [Binary Tree Level Order Traversal II](https://leetcode.com/problems/binary-tree-level-order-traversal-ii)                                                |                 [![Java](assets/java.png)](src/BinaryTreeLevelOrderTraversalII.java) [![Python](assets/python.png)](python/binary_tree_level_order_traversal_ii.py)                  |                                                                                                                                   |
 | 108  | [Convert Sorted Array To Binary Search Tree](https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree)                                    |           [![Java](assets/java.png)](src/ConvertSortedArrayToBinarySearchTree.java) [![Python](assets/python.png)](python/converted_sorted_array_to_binary_search_tree.py)           |                                                                                                                                   |
 | 109  | [Convert Sorted List to Binary Search Tree](https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree)                                      |                                                                                                                                                                                      |                                                                                                                                   |

Diff for: src/ConstructBinaryTreeFromInorderAndPostorderTraversal.java

@@ -0,0 +1,34 @@
+// https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal
+// T: O(n)
+// S: O(n)
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class ConstructBinaryTreeFromInorderAndPostorderTraversal {
+    Map<Integer, Integer> inOrderIndices;
+    int[] postOrder;
+
+    public TreeNode buildTree(int[] inorder, int[] postorder) {
+        inOrderIndices = getIndicesMap(inorder);
+        this.postOrder = postorder;
+        return buildTreePostIn(0, inorder.length - 1, 0, postorder.length - 1);
+    }
+
+    private TreeNode buildTreePostIn(int is, int ie, int ps, int pe){
+        if (ps > pe || is > ie) return null;
+        final TreeNode root = new TreeNode(postOrder[pe]);
+        int ri = inOrderIndices.get(postOrder[pe]);
+        root.left = buildTreePostIn(is, ri-1, ps, ps+ri-is-1);
+        root.right = buildTreePostIn(ri+1, ie, ps+ri-is, pe-1);
+        return root;
+    }
+
+    private Map<Integer, Integer> getIndicesMap(int[] array) {
+        final Map<Integer, Integer> result = new HashMap<>(array.length);
+        for (int index = 0 ; index < array.length ; index++) {
+            result.put(array[index], index);
+        }
+        return result;
+    }
+}

Diff for: src/ConstructBinaryTreeFromPreorderAndInorderTraversal.java

@@ -15,7 +15,7 @@ public TreeNode buildTree(int[] preorder, int[] inorder) {
     }
 
     private TreeNode buildTree(int[] preOrder, int[] inOrder, int start, int end) {
-        if (start >= end) return null;
+        if (start == end) return null;
 
         int inOrderIndex = inOrderIndices.get(preOrder[preOrderIndex]);
         final TreeNode root = new TreeNode(preOrder[preOrderIndex++]);

Diff for: src/HelloWorld.java

@@ -4,7 +4,20 @@
 
 public class HelloWorld {
     public static void main(String[] args) {
-        List<Integer> list = new ArrayList<>();
-        Collections.reverse(list);
+//        ConstructBinaryTreeFromInorderAndPostorderTraversal c = new ConstructBinaryTreeFromInorderAndPostorderTraversal();
+//        TreeNode treeNode = c.buildTree(new int[] {9, 3, 15, 20, 7}, new int[] {9, 15, 7, 20, 3});
+//        TreePrinter.print(treeNode);
+//
+//        c = new ConstructBinaryTreeFromInorderAndPostorderTraversal();
+//        treeNode = c.buildTree(new int[] {2, 1}, new int[] {2, 1});
+//        TreePrinter.print(treeNode);
+//
+//        c = new ConstructBinaryTreeFromInorderAndPostorderTraversal();
+//        treeNode = c.buildTree(new int[] {-1}, new int[] {-1});
+//        TreePrinter.print(treeNode);
+
+        ConstructBinaryTreeFromInorderAndPostorderTraversal c = new ConstructBinaryTreeFromInorderAndPostorderTraversal();
+        TreeNode treeNode = c.buildTree(new int[] {1, 2, 3, 4}, new int[] {2, 1, 3, 4});
+        TreePrinter.print(treeNode);
     }
 }