construct bst from in order and postorder traversal v-2.java

/**
 * 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 == null || postorder == null)
            return null;
        if (inorder.length != postorder.length)
            return null;
        int len = inorder.length;
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < len; i++)
            map.put(inorder[i], i);
        return buildTree(inorder, 0, len - 1, postorder, 0, len - 1, map);
    }
    
    private TreeNode buildTree(int[] inorder, int lo1, int hi1, int[] postorder, int lo2, int hi2, HashMap<Integer, Integer> map) {
        if (lo1 > hi1)
            return null;
        TreeNode node = new TreeNode(postorder[hi2]);
        int index = map.get(postorder[hi2]);
        int left = index - lo1, right = hi1 - index;
        node.left = buildTree(inorder, lo1, index - 1, postorder, lo2, lo2 + left - 1, map);
        node.right = buildTree(inorder, index + 1, hi1, postorder, lo2 + left, hi2 - 1, map);
        return node;
    }
}