Leetcode 106. Construct Binary Tree from Inorder and Postorder Traversal

Answer:

 /**  
  * Definition for a binary tree node.  
  * public class TreeNode {  
  *   public int val;  
  *   public TreeNode left;  
  *   public TreeNode right;  
  *   public TreeNode(int x) { val = x; }  
  * }  
  */  
 public class Solution {  
   public TreeNode BuildTree(int[] inorder, int[] postorder) {  
     return helper(inorder, 0, inorder.Length - 1, postorder, 0, postorder.Length - 1);  
   }  
   TreeNode helper(int[] inorder, int istart, int iend, int[] postorder, int pstart, int pend)  
   {  
     for (int i = istart; i <= iend; i++)  
     {  
       // Find the root from inorder array  
       if (inorder[i] == postorder[pend])  
       {  
         TreeNode root = new TreeNode(inorder[i]);  
         root.left = helper(inorder, istart, i - 1, postorder, pstart, pstart + i - 1 - istart);  
         root.right = helper(inorder, i + 1, iend, postorder, pend-1 - (iend-i-1), pend - 1);  
         return root;  
       }  
     }  
     return null;  
   }  
 }