Binary Tree Level Order Traversal II

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        vector<vector<int> > vec;

        if(root == NULL) return vec;
        queue<TreeNode *> crt, wait;
        crt.push(root);
        while(crt.size() > 0){
            int num = crt.size();
            vector<int> arr;
            while(num--){
                TreeNode *tmp = crt.front();
                if(tmp->left) wait.push(tmp->left);
                if(tmp->right) wait.push(tmp->right);
                arr.push_back(tmp->val);
                crt.pop();
            }
            vec.insert(vec.begin(), arr);
            int n = wait.size();
            while(n--){
                TreeNode *tmp = wait.front();
                wait.pop();
                crt.push(tmp);
            }
        }
        return vec;
    }
};