层序遍历二叉树

利用队列记录每一层节点个数

ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        if (pRoot == null) {
            return res;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(pRoot);
        while (queue.isEmpty() == false) {
            int curNum = queue.size();   // 获取每层的节点个数
            ArrayList<Integer> tmp = new ArrayList<>();
            for (int i = 0; i < curNum; i++) {
                TreeNode cur = queue.poll();
                tmp.add(cur.val);
                if (cur.left != null) {
                    queue.offer(cur.left);
                }
                if (cur.right != null) {
                    queue.offer(cur.right);
                }
            }
            res.add(tmp);
        }
        return res;
    }