Binary Tree Postorder Traversal

 1 class Solution {
 2 public:
 3     vector<int> postorderTraversal(TreeNode *root) {
 4         if(root->left == NULL && root->right == NULL)
 5         {
 6             v1.push_back(root->val);
 7             return v1;
 8         }
 9         if(root->left != NULL)
10         postorderTraversal(root->left);
11         if(root->right != NULL)
12         postorderTraversal(root->right);
13         v1.push_back(root->val);
14         return v1;
15     }
16 };