Preorder Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void preorder(TreeNode* root, vector<int>&ans){
        if(root==nullptr) return ;

        stack<TreeNode*>st;
        st.push(root);

        while(!st.empty()){
            TreeNode* curr = st.top();
            st.pop();
            ans.push_back(curr->val);
            if(curr->right) st.push(curr->right);
            if(curr->left) st.push(curr->left);
        }

        return;
    }

    vector<int> preorderTraversal(TreeNode* root) {
        vector<int>ans;

        preorder(root,ans);

        return ans;    
    }
};