#include <bits/stdc++.h>
using namespace std;

// define tree node structure
struct TreeNode {
    // store value
    int val;
    // left child pointer
    TreeNode *left;
    // right child pointer
    TreeNode *right;
    // constructor to initialize node
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

// solution class holding state
class Solution {
public:
    // previous node pointer for reverse preorder linking
    TreeNode* prev = nullptr;

    // flatten tree to right-skewed linked list (preorder sequence)
    void flatten(TreeNode* root) {
        // return if node is null
        if (root == nullptr) return;
        // flatten right subtree first
        flatten(root->right);
        // flatten left subtree next
        flatten(root->left);
        // connect current node's right to previously processed node
        root->right = prev;
        // nullify left pointer
        root->left = nullptr;
        // update previous to current
        prev = root;
    }
};

// print preorder traversal
void printPreorder(TreeNode* root) {
    // return on null
    if (!root) return;
    // print current node
    cout << root->val << " ";
    // traverse left subtree
    printPreorder(root->left);
    // traverse right subtree
    printPreorder(root->right);
}

// print nodes following right pointers
void printFlattenTree(TreeNode* root) {
    // return on null
    if (!root) return;
    // print current node
    cout << root->val << " ";
    // move to right child
    printFlattenTree(root->right);
}

// program entry point
int main() {
    // construct sample tree
    TreeNode* root = new TreeNode(1);
    root->left = new TreeNode(2);