[leetcode] 144. 二叉树的前序遍历

144. 二叉树的前序遍历

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        dfs(root, list);
        return list;
    }

    public void dfs(TreeNode node, List<Integer> ans) {
        ans.add(node.val);
        if (node.left != null) dfs(node.left, ans);
        if (node.right != null) dfs(node.right, ans);
    }
}class Solution {
    public List