Binary Tree Preorder Traversal.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;

        preorder(res, root);

        return res;
    }

    void preorder(vector<int>& res, TreeNode* root){
    	if(root == NULL)
    		return;

    	res.push_back(root->val);

    	preorder(res, root->left);
    	preorder(res, root->right);

    }
};