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;    }};