/**
 * Definition for a binary tree node.
 */
#include <vector>
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 {
private:
    std::vector<int> traversal;
public:
    std::vector<int> postorderTraversal(TreeNode* root) {
        if (root != nullptr) {
            postorderTraversal(root->left);
            postorderTraversal(root->right);
            traversal.push_back(root->val);
        }
        return traversal;
    }
};