Binary Tree Preorder Traversal

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution
{
public:
    void PreOrder(TreeNode* root,vector<int>&vec)
    {
        if(root!=NULL)
        {
            vec.push_back(root->val);
            PreOrder(root->left,vec);
            PreOrder(root->right,vec);
        }
    }
    vector<int> preorderTraversal(TreeNode *root)
    {
         vector<int>vec;
         PreOrder(root,vec);
         return vec;
    }
};