Leetcode: Construct Binary Tree from Inorder and Postorder Traversal

Given inorder and postorder traversal of a tree, construct the binary 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) {
if (inorder.length==0 || postorder.length==0 || inorder.length!=postorder.length) {
return null;
}
int len = inorder.length;
HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
for (int i=0; i<inorder.length; i++) {
map.put(inorder[i], i);
}
return helper(postorder, 0, len-1, inorder, 0, len-1, map);
}

public TreeNode helper(int[] postorder, int posL, int posR, int[] inorder, int inL, int inR, HashMap<Integer, Integer> map) {
if (posL>posR || inL>inR) {
return null;
}
TreeNode root = new TreeNode(postorder[posR]);
int index = map.get(postorder[posR]);
root.left = helper(postorder, posL, posL+index-1-inL, inorder, inL, index-1, map);
root.right = helper(postorder, posL+index-inL, posR-1, inorder, index+1, inR, map);
return root;
}
}