leetcode---Construct Binary Tree from Inorder and Postorder Traversal

Given inorder and postorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        int length = inorder.length;
        return buildTreeHelp(inorder, postorder, 0, length - 1, 0, length - 1);
    }
    public TreeNode buildTreeHelp(int[] inorder, int[] postorder, int startI, int endI, int startP, int endP){
        TreeNode root = null;
        if(startI <= endI){
            root = new TreeNode(postorder[endP]);
            int rootPosition = startI;
            int newendP = startP;
            for(int i = startI; i <= endI; ++i){
                if(inorder[i] == postorder[endP]){
                    rootPosition = i;
                    break;
                }
                else
                    ++newendP;
            }
            root.left = buildTreeHelp(inorder, postorder, startI, rootPosition - 1, startP, newendP - 1);
            root.right = buildTreeHelp(inorder, postorder, rootPosition + 1, endI, newendP, endP - 1);
        }
        return root;
    }
}