binaryPreOrderTraversal

// problem 145
#include <vector>
using namespace std;

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:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res = {};
        if(root){
            res.push_back(root->val);

            vector<int> left = preorderTraversal(root->left);
            vector<int> right = preorderTraversal(root->right);
            
            res.insert(res.end(), left.begin(), left.end());
            res.insert(res.end(), right.begin(), right.end());
        }
        return(res);
        
    }
};