LeetCode 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 {
private:
    vector<int> retVec; // the traversal sequence of output
public:
    void recursiveTraverse(TreeNode *root){
        if(root == NULL)
            return;
        retVec.push_back(root->val);
        recursiveTraverse(root->left);
        recursiveTraverse(root->right);
    }
    void iterTraverse(TreeNode *root){
        if(root==NULL)
            return;
        stack<TreeNode*> st;
        TreeNode* temp;
        st.push(root);
        while(!st.empty())
        {
            temp=st.top();
            retVec.push_back(temp->val);
            st.pop();
            if(temp->right!=NULL)
                st.push(temp->right);
            if(temp->left!=NULL)
                st.push(temp->left);
            
        }
    }
    vector<int> preorderTraversal(TreeNode *root) {
     //   recursiveTraverse(root);
        iterTraverse(root);
        return retVec;
    }
};