text: /**
   * Definition for a binary tree node.
   * 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> ret;
  
      void traverse(TreeNode* root) {
          if(root == nullptr) return;
          traverse(root->left);
          traverse(root->right);
          ret.push_back(root->val);
      }
  
      vector<int> postorderTraversal(TreeNode* root) {
          traverse(root);
          return ret;
      }
  };