LC94–二叉树的中序遍历—144. 二叉树的前序遍历

输入：root = [1,null,2,3]
输出：[1,3,2]

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

    void dfs(ArrayList<Integer> list, TreeNode root) {
   
        if (root == null) {
   
            return;
        }
        dfs(list, root.left);
        list.add(root.val);
        dfs(list, root.right);

    }
}

输入：root = [1,null,2,3]
输出：[1,2,3]

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

    void dfs(ArrayList<Integer> list, TreeNode root) {
   
        if (root == null) {
   
            return;
        }
         list.add(root.val);
        dfs(list, root.left);
       
        dfs(list, root.right);

    }
}