Construct Binary Tree from Inorder and Postorder Traversal

 1 /**
 2  * Definition of TreeNode:
 3  * public class TreeNode {
 4  *     public int val;
 5  *     public TreeNode left, right;
 6  *     public TreeNode(int val) {
 7  *         this.val = val;
 8  *         this.left = this.right = null;
 9  *     }
10  * }
11  */
12  
13  
14 public class Solution {
15     /**
16      *@param inorder : A list of integers that inorder traversal of a tree
17      *@param postorder : A list of integers that postorder traversal of a tree
18      *@return : Root of a tree
19      */
20     public TreeNode buildTree(int[] inorder, int[] postorder) {
21         if (inorder == null || postorder == null || postorder.length == 0 || inorder.length != postorder.length) return null;
22         
23         return buildTree(inorder, 0, postorder, 0, inorder.length);
24         
25     }
26     
27     public TreeNode buildTree(int[] inorder, int inStart, int[] postorder, int postStart, int length) {
28         if (length <= 0) return null;
29         TreeNode root = new TreeNode(postorder[postStart + length - 1]);
30 
31         int index = findIndex(inorder, root.val);
32         root.left = buildTree(inorder, inStart, postorder, postStart, index - inStart);
33         root.right = buildTree(inorder, index + 1, postorder, postStart + (index - inStart), length - (index - inStart) - 1);
34 
35         return root;    
36         
37     }
38 
39     public int findIndex(int[] inorder, int val) {
40         for (int i = 0; i < inorder.length; i++) {
41             if (inorder[i] == val) {
42                 return i;
43             }
44         }
45         return -1;
46     }
47 }