145. Binary Tree Postorder Traversal

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> list;
public List<Integer> postorderTraversal(TreeNode root) {
list = new ArrayList<Integer>();
postorderTraversal2(root);
return list;

}

public void postorderTraversal2(TreeNode root) {
if(root == null){
return;
}
postorderTraversal2(root.left);
postorderTraversal2(root.right);
list.add(root.val);
}
}