从上到下打印二叉树II

题解1：

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> res = new ArrayList<>();
        if(root != null) queue.add(root);
        while(!queue.isEmpty()) {
            List<Integer> tmp = new ArrayList<>();
            for(int i = queue.size(); i > 0; i--) {
                TreeNode node = queue.poll();
                tmp.add(node.val);
                if(node.left != null) queue.add(node.left);
                if(node.right != null) queue.add(node.right);
            }
            res.add(tmp);
        }
        return res;
    }
}

题解2：

class Solution {
    List<List<Integer>> res = new ArrayList();
    public List<List<Integer>> levelOrder(TreeNode root){
        recur(root,0);
        return res;
    }
    public void recur(TreeNode root,int k){
        if(root != null){
            if(res.size() <= k)
                res.add(new ArrayList());
            res.get(k).add(root.val);
            recur(root.left,k+1);
            recur(root.right,k+1);
        }
    }
}