Preorder Traversal

• Time : $O(n)$
• Space : $O(n)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
void helper(TreeNode* node, vector<int>& res) {
    if(!node) return;
    res.push_back(node->val);
    helper(node->left, res);
    helper(node->right, res);
}
vector<int> Solution::preorderTraversal(TreeNode* A) {
    vector<int> res;
    helper(A,res);
    return res;
}