102. 二叉树的层序遍历

层序遍历框架固定，先写出固定的框架；然后做些修改

class Solution {
    public List> levelOrder(TreeNode root) {
        if(root == null) return new ArrayList();
        TreeNode node = root;

        ArrayList> result = new ArrayList<>();
        ArrayList level = new ArrayList<>();

        Queue que = new linkedList<>();
        que.offer(node);
        int levelSize = 1;

        while(!que.isEmpty()){
            TreeNode poll = que.poll();
            level.add(poll.val);
            levelSize -= 1;

            if(poll.left != null){
                que.offer(poll.left);
            }
            if(poll.right != null){
                que.offer(poll.right);
            }

            if(levelSize == 0){
                levelSize = que.size();
                result.add(level);
                level = new ArrayList();
            }
        }

        return result;
    }
}