Binary Tree Preorder Traversal

트리의 root 노드를 받고 preoder 순으로 트리를 방문하며,

방문한 노드들의 값을 순서대로 저장한 벡터를 반환하는 문제

/**
 * 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 SearchDFS(TreeNode* root, vector<int> &result)
    {
        if (root == NULL)
        {
            return;
        }

        result.push_back(root->val);
        SearchDFS(root->left, result);
        SearchDFS(root->right, result);

        return;
    }

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

        SearchDFS(root, result);

        return result;
    }
};