【LeetCode OJ】Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        inOrder(root);
        return list; 
    }

	public List<Integer> list = new ArrayList<Integer>();
	public void inOrder(TreeNode root){
		if(root == null)return;
		inOrder(root.left);
		list.add(root.val);
		inOrder(root.right);
	}
}