Algotithm, Binary tree, preorder search (recursive)

recursive_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> out;
        
        out.push_back(root->val);
        
        if (root->left == NULL){
            
            if (root->right == NULL){
                
                return out;
                
            }else{
                
                vector<int> reout = preorderTraversal(root->right);
            
                out.insert(out.end(), reout.begin(), reout.end());    
                
            }
            
        }else{
            
            vector<int> reout = preorderTraversal(root->left);
            
            out.insert(out.end(), reout.begin(), reout.end());
            
        }
        
        return out;
    }
};