/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

class Solution {

    public TreeNode buildTree(int[] inorder, int[] postorder) {

        Map<Integer, Integer> mymap = new HashMap<>();

        for (int i = 0; i < inorder.length; i++) {

            mymap.put(inorder[i], i);

        }

        return helper(0, inorder.length - 1, 0, postorder.length - 1, postorder, mymap);

    }

        private TreeNode helper(int inLeft, int inRight, int postLeft, int postRight, int[] postorder, Map<Integer, Integer> mymap) {

        if (inLeft > inRight) {

            return null;

        }

        TreeNode cur = new TreeNode(postorder[postRight]);

        int index = mymap.get(postorder[postRight]);

        int leftSize = index - inLeft;

        // postRight for left just add up leftSize

        cur.left = helper(inLeft, index - 1, postLeft, postLeft + leftSize - 1, postorder, mymap);

        cur.right = helper(index + 1, inRight, postLeft + leftSize, postRight - 1, postorder, mymap);

        return cur;

    }

}