Leetcode: Construct Binary Tree from Inorder and Postorder Traversal

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

与Construct Binary Tree from Inorder and Preorder Traversal问题非常类似，唯一区别在于这一次确定root的位置由post traversal来确定，为最后一个元素。

/**
 * 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;
    }
}