Binary Tree Level Order Traversal

/**
 * 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 {
private:
    vector<vector<int>> ret;
public:
    void traversal(int dep, TreeNode *root) {

        if(root == NULL)
            return;

        if(ret.size() > dep) 
            ret[dep].push_back(root->val);
        else {
            vector<int> tmp;
            tmp.push_back(root->val);
            ret.push_back(tmp);
        }
        traversal(dep + 1,root->left);
        traversal(dep + 1,root->right);
    }
    vector<vector<int>> levelOrder(TreeNode* root) {
           traversal(0, root);
           return ret;
    }
};