222. Count Complete Tree Nodes

Given a complete binary tree, count the number of nodes.

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

代码如下：（超时）

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public int countNodes(TreeNode root) {
         List<TreeNode> list=PreOrder(root);
         return list.size();

     }
     public List<TreeNode> PreOrder(TreeNode root){
         List<TreeNode> list=new ArrayList<>();
         if(root==null)
         return list;

         list.add(root);
         if(root.left!=null)
         list.addAll(PreOrder(root.left));
         if(root.right!=null)
         list.addAll(PreOrder(root.right));
         return list;
     }
 }