解题思路

层次遍历 广度优先搜索

 public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        boolean reverse = false;
        while (!queue.isEmpty()) {
            List<Integer> list = new ArrayList<>();
            int cnt = queue.size();
            while (cnt-- > 0) {
                TreeNode node = queue.poll();
                if (node == null)
                    continue;
                list.add(node.val);
                queue.add(node.left);
                queue.add(node.right);
            }
            if (reverse)
                Collections.reverse(list);
            reverse = !reverse;
            if (list.size() != 0)
                ret.add(list);
        }
        return ret;
    }