144. 二叉树的前序遍历

Java代码：递归

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    preOrder(root, res);
    return res;
}
private void preOrder(TreeNode root, List<Integer> res) {
    if (root == null) {
        return;
    }
    res.add(root.val);
    preOrder(root.left, res);
    preOrder(root.right, res);
}