/**
 * 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 {
public:
    // TODO: Make this iterative instead of recursive.
    std::vector<int> postorderTraversal(TreeNode* root) {
        //TODO
    }
};