LeetCode 144：Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes’ values.

Example:

Input: [1,null,2,3]

Output: [1,2,3]

C++
vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        DFS(res,root);
        return res;
    }
    void DFS(vector<int>& res,TreeNode* root)
    {
        if(root != nullptr)
        {
            res.push_back(root->val);
            DFS(res,root->left);
            DFS(res,root->right);
        }
    }