Construct Binary Tree from Inorder and Postorder Traversal

buildTree.java

/*
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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(inorder.length == 0) return null;
        
        return buildTreeHelper(inorder, postorder, 0, inorder.length-1, 0, postorder.length-1);
    }
    
    public TreeNode buildTreeHelper(int[] inorder, int[] postorder, int in_s, int in_e, int po_s, int po_e) {
        if(po_e < po_s) return null;
        int end = postorder[po_e];
        po_e--;
        TreeNode root = new TreeNode(end);
        int len = 0;
        for(int i = in_e; i > in_s; i--) {
            if(inorder[i] == end) break;
            len++;
        }

        root.left = buildTreeHelper(inorder, postorder, in_s, in_e-len-1, po_s, po_e-len);
        root.right = buildTreeHelper(inorder, postorder, in_e-len+1, in_e, po_e-len+1, po_e);
        return root;
    }
}