144. 二叉树前序遍历

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

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