输入: [1,null,2,3]  
   1
    \
     2
    /
   3 
​
输出: [1,2,3]

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if(root != null){
             list.add(root.val);
             list.addAll(preorderTraversal(root.left));
             list.addAll(preorderTraversal(root.right));
        }
        return list;
    }
}

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if(root == null){
            return list;
        }
        LinkedList<TreeNode> stack = new LinkedList();
        stack.push(root);
        while(!stack.isEmpty()){
            root = stack.pop();    
            list.add(root.val);
            if(root.right!=null){
                stack.push(root.right);
            }
            if(root.left!=null){
                stack.push(root.left);
            }
        }
        return list;
    }
}