样例
输入如下图所示二叉树[8, 12, 2, null, null, 6, null, 4, null, null, null]
    8
   /   12  2
     /
    6
   /
  4

输出：[[8], [12, 2], [6], [4]]

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> res;
    queue<TreeNode*>  que;

    void bfs()
    {

        while(!que.empty()){
            vector<int> lineVec;
            while(!que.empty() && NULL != que.front() ){
                TreeNode* p = que.front();
                que.pop();
                lineVec.push_back(p->val);
                if(p->left!=NULL)
                    que.push(p->left);
                if(p->right!=NULL)
                    que.push(p->right);
            }
            res.push_back(lineVec);
            if(!que.empty()){
                que.pop();
            }
            if(!que.empty()){
                que.push(NULL);
            }
        }
    }


    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        if(root == NULL) return res;
        que.push(root); que.push(NULL);
        bfs();

        return res;
    }
};

作者：defddr
链接：https://www.acwing.com/solution/acwing/content/3661/
来源：AcWing
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