84. Construct Binary Tree from Inorder and Postorder Traversal

/**
 * 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) return null;
        return traversal(inorder, postorder, 0, inorder.length-1, postorder.length-1);
    }
   
    public TreeNode traversal(int[] inorder, int[] postorder, int ist, int iend, int pst) {
        if (ist>iend) return null;
        TreeNode res = new TreeNode(postorder[pst]);
        int mid = 0;
        for (int i=ist; i<=iend; i++) {
            if (inorder[i]==res.val) {
                mid = i;
                break;
            }
        }
        res.left = traversal(inorder, postorder, ist, mid-1, pst-1-iend+mid);
        res.right = traversal(inorder, postorder, mid+1, iend, pst-1);
        return res;
    }
}