题解 | #求二叉树的层序遍历#

BFS

    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        ArrayList<ArrayList<Integer>> list = new ArrayList<>();
         if(root == null) return list;
         Queue<TreeNode> queue = new LinkedList<>();
         queue.offer(root);
         while(!queue.isEmpty()){
             int size = queue.size();
             ArrayList<Integer> child = new ArrayList<>();
             while(size -- != 0){
                 TreeNode node = queue.poll();
                 child.add(node.val);
                 if(node.left != null) queue.offer(node.left);
                 if(node.right != null) queue.offer(node.right);
             }
             list.add(child);
         }
         return list;
    }

DFS

    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        ArrayList<ArrayList<Integer>> list = new ArrayList<>();
        int h = 0;
        helper(list,h,root);
        return list;
    }
    
    private void helper(ArrayList<ArrayList<Integer>> list,int h,TreeNode root){
        if(root == null) return ;
        if(h >= list.size()){
            list.add(new ArrayList<>());
        }
        list.get(h).add(root.val);
        helper(list,h+1,root.left);
        helper(list,h+1,root.right);
    }