Binary Tree Inorder Traversal

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

   1
    
     2
    /
   3

return [1,3,2].

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    List<Integer> re_list = new ArrayList<Integer>();
    //中序遍历二叉树
    public void IFS(TreeNode N) {
        if(N!=null) {
            if(N.left!=null) IFS(N.left);
            re_list.add(N.val);
            if(N.right!=null) IFS(N.right);
        }
    } 
    
    public List<Integer> inorderTraversal(TreeNode root) {
        IFS(root);
        return re_list;
    }
}