剑指 Offer 32 - II. 从上到下打印二叉树 II

题目链接：
https://leetcode.cn/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof/description/?favorite=xb9nqhhg
代码：

class Solution {
public:
vector<vector<int>>ans;

    vector<vector<int>> levelOrder(TreeNode* root) {
        if(root==NULL)
        {
            return ans;
        }
       queue<TreeNode*>q;
       q.push(root);
       while(!q.empty())
       {
           int num=q.size();
           ans.push_back(vector<int>());
           for(int i=0;i<num;i++)
           {
               TreeNode *top=q.front();
               q.pop();
               ans.back().push_back(top->val);
               if(top->left) q.push(top->left);
               if(top->right) q.push(top->right);
           }

       }
       return ans;
    }
};