Doubly linked list in C. (dynamic/pointer-based tree) https://en.wikipedia.org/wiki/Doubly_linked_list

list.c

#include <stdio.h>
#include <stdlib.h>
#define pf printf

#define oo 0x7FFFFFFF
#define nodeptr struct node_t*

struct node_t {
	int val;
	nodeptr rr;
	nodeptr ll;
};

nodeptr new_node(int val) {
	nodeptr x
	= (nodeptr)malloc(sizeof(struct node_t));
	x->val = val; x->ll = NULL; x->rr = NULL;
	return x;
}

nodeptr list_ll;
nodeptr list_rr;

void new_list(void) {
	list_ll = new_node(-oo);
	list_rr = new_node(+oo);
	list_ll->rr = list_rr;
	list_rr->ll = list_ll;
}

////////////////////////////////////////////////////////////////////////////////

#define PUSH(x, y, _val, _inf)        \
	list_##x->val = _val;         \
	list_##x->x = new_node(_inf); \
	list_##x->x->y = list_##x;    \
	list_##x = list_##x->x;       \

#define POP(x, y, _inf)               \
	list_##x = list_##x->y;       \
	list_##x->x = NULL;           \
	list_##x->val = _inf;         \

void push_left(int x)
	{ PUSH(ll, rr, x, -oo); }

void pop_left(void)
	{ POP(ll, rr, -oo); }

void push_right(int x)
	{ PUSH(rr, ll, x, +oo); }

void pop_right(void)
	{ POP(rr, ll, +oo); }

////////////////////////////////////////////////////////////////////////////////

#define DEBUG(x, y, _inf)                         \
	nodeptr t = list_##x;                     \
	while (t->val != _inf) {                  \
		if (t->val == +oo ||              \
			t->val == -oo) goto next; \
		pf("[%d]", t->val);               \
		next: t = t-> y; }                \

void debug_left(void)
	{ DEBUG(ll, rr, +oo); }

void debug_right(void)
	{ DEBUG(rr, ll, -oo); }

////////////////////////////////////////////////////////////////////////////////

int main() {
	new_list();

	// (1)
	push_left(5);
	push_left(2);
	push_left(1200);
	push_right(10);
	push_right(20);
	push_right(11);
	push_right(12);
	push_right(200);
	push_right(15);

	// (2)
	pop_right();
	pop_left();
	pop_left();
	push_right(25);
	push_right(100);

	// (1) [1200][2][5][10][20][11][12][200][15]
	// (2)   X    X                          X   + [25][100]

	pf("=== @1: CASE ===\n");
	debug_left(); pf("\n");

	pf("=== @2: CASE ===\n");
	debug_right(); pf("\n");

	return 0;
}

output.txt

=== @1: CASE ===
[5][10][20][11][12][200][25][100]
=== @2: CASE ===
[100][25][200][12][11][20][10][5]