模版

 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrder(TreeNode* root) {
        // write code here
        vector<vector<int> > res;
        if(!root) return res;
        queue<TreeNode* > qu;
        qu.push(root);
        while(!qu.empty()){
            int n = qu.size();
            vector<int> vec;
            while(n--){
                TreeNode *node = qu.front();
                qu.pop();
                vec.emplace_back(node->val);
                if(node->left) qu.push(node->left);
                if(node->right) qu.push(node->right);
            }
            res.emplace_back(vec);
        }
        return res;
    }
};