LintCode 68---Binary Tree Postorder Traversal

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: Postorder in ArrayList which contains node values.
     */
   List<Integer> list = new ArrayList<>();
     public List<Integer> postorderTraversal(TreeNode root) {
         hou(root);
         return list;
        }
     public void hou(TreeNode root) {
         if(root == null) {
             return;
         }
         hou(root.left);
         hou(root.right);
         list.add(root.val);
     }
}