Binary Tree Level Order Traversal--leetcode

/**
 * 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> levelOrder(TreeNode* root) {
        vector> res;
        if (root == NULL) { return res; }
        vector list;
        list.push_back(root);
        vector temp2;
        temp2.push_back(root->val);
        res.push_back(temp2);
        while (list.size()!=0)
        {
            vector temp;
            vector temp2;
            for (size_t i = 0; i < list.size(); i++)
            {
                if (list[i]->left!=NULL) {
                    temp.push_back(list[i]->left);
                    temp2.push_back(list[i]->left->val);
                }
                if (list[i]->right != NULL) {
                    temp.push_back(list[i]->right);
                    temp2.push_back(list[i]->right->val);
                }
            }
            list = temp;
            if(temp.size()!=0){
                res.push_back(temp2);
            }

        }
        return res;
    }
};