class Solution {
public:
    void visit(TreeNode* t,vector<int> &a)
    {
        if(t==NULL)
        {
           return;
        }
        a.push_back(t->val);
        visit(t->left,a);
        visit(t->right,a);
    }
    vector<int> preorderTraversal(TreeNode* root) {
    vector<int> a;
    if(root==NULL)
    {
        return a;
    }
    else
    {
        a.push_back(root->val);
        visit(root->left,a);
        visit(root->right,a);
    }
    return a;
    }
};