BT_Preorder_Rec.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;
        rec(res, root);
        return res;
    }
    void rec(vector<int> &res, TreeNode* root){
        if(root == NULL) return;
        //We first record the root.
        res.push_back(root->val);
        rec(res, root->left);
        rec(res, root->right);
    }
};