без принтов улучшено

#include <iostream>
using namespace std;

struct Node
{
    int key;
    int idx;
    Node* left;
    Node* right;
    Node* parent;
};

void find_parent(int* l, int* r, int idx, Node* tree)
{

    int idx_left = *l;
    int idx_right = *r;
    bool is_greater = tree[0].key <= tree[idx].key;
    Node* curr;

    is_greater ? curr = &tree[idx_right] : curr = &tree[idx_left];

    if (curr->key <= tree[idx].key && curr->right == NULL)
    {
        tree[idx].parent = curr;
        curr->right = &tree[idx];
        is_greater ? * r = idx : * l = idx;
    }

    else if (curr->key > tree[idx].key)
    {
        is_greater ? * r = tree[idx_right].left->idx : * l = tree[idx_left].left->idx;
        find_parent(l, r, idx, tree);
    }

    else if (curr->right != NULL)
    {
        is_greater ? * r = tree[idx_right].right->idx : * l = tree[idx_left].right->idx;
        find_parent(l, r, idx, tree);
    }

    return;
}


void f(int* idx_left, int* idx_right, int k, Node* tree, int n)
{

    while (k < n - 1 & tree[k + 1].key <= tree[k].key)
    {
        tree[k].left = &tree[k + 1];
        tree[k + 1].parent = &tree[k];
        k++;
    }

    if (k + 1 < n)
        find_parent(idx_left, idx_right, k + 1, tree);

    if (k + 2 < n)
        f(idx_left, idx_right, k + 1, tree, n);
}

void preorderTraversal(Node* x)
{
    if (x != NULL)
    {
        cout << x->key << " ";
        preorderTraversal(x->left);
        preorderTraversal(x->right);
    }
    return;
}

void inorderTraversal(Node* x)
{
    if (x != NULL)
    {
        inorderTraversal(x->left);
        cout << x->key << " ";
        inorderTraversal(x->right);
    }
    return;
}

void postorderTraversal(Node* x)
{
    if (x != NULL)
    {
        postorderTraversal(x->left);
        postorderTraversal(x->right);
        cout << x->key << " ";
    }
    return;
}

void print_tree(Node* tree, int n)
{
    cout << endl;
    for (int i = 0; i < n; ++i)
    {
        cout << "i= " << tree[i].key << ": ";
        if (tree[i].left != NULL)
            cout << tree[i].left->key << " ";
        if (tree[i].right != NULL)
            cout << tree[i].right->key << " ;";
        cout << endl;
    }
    cout << endl;
}

int main()
{
    int n;
    cin >> n;

    int idx_left = 0, idx_right = 0;
    Node tree[n];

    for (int i = 0; i < n; ++i)
    {
        tree[i].right = NULL;
        tree[i].left = NULL;
        tree[i].parent = NULL;
        tree[i].idx = i;
        cin >> tree[i].key;
    }


    f(&idx_left, &idx_right, 0, tree, n);

    print_tree(tree, n);

    cout << endl;
    preorderTraversal(&tree[0]);
    cout << endl;
    postorderTraversal(&tree[0]);
    cout << endl;
    inorderTraversal(&tree[0]);
    cout << endl;

    return 0;
}