Language: C

#include <stdio.h>
#include <ctype.h>
#include <string.h>

typedef struct node{
  int key;
  struct node *next;
  struct node *prev;
}NODE,*PNODE;???//PNODE???malloc???????????????

PNODE root;

void init(void) {
  root = malloc(sizeof(NODE)); 
    root->next = root;
    root->prev = root;
}

void insert(key) {
    PNODE t = malloc(sizeof(NODE));
    t->key = key;
    t->next = root->next;
    t->prev = root;
    root->next->prev = t;
    root->next = t;
}
 
void deleteNode(PNODE node) {
    if (node == root) {
        return;
    }
    node->next->prev = node->prev;
    node->prev->next = node->next;
    free(node);
}
 
void delete(key) {
    PNODE cur = root->next;
    while (cur != root) {
        if (cur->key == key) {
            deleteNode(cur);
            return;
        }
        cur = cur->next;
    }
}
 
void deleteFirst(void) {
    deleteNode(root->next);
}
 
void deleteLast(void) {
    deleteNode(root->prev);
}

void printNodes(void) {
    PNODE cur = root->next;
    while (1) {
        printf("%d", cur->key);
        cur = cur->next;
        if (cur != root) {
            putchar(' ');
        } else {
            break;
        }
    }
    putchar('\n');
}

void release(void) {
    PNODE tmp, cur = root->next;
    while (cur != root) {
        tmp = cur;
        cur = cur->next;
        deleteNode(tmp);        
    }
    free(root);
} 

int main(){
  char cmd[100];
  int n, i, f;
  int key;

  init();

  scanf("%d",&n);
  
  f = 0;
    while (scanf("%s", cmd) > 0) {
        if (cmd[0] == 'i') {
            f = 1;
        } else if (cmd[0] == 'd') {
            switch (cmd[6]) {
            case '\0':
                f = 2;
                break;
            case 'F':
                deleteFirst();
                break;
            case 'L':
                deleteLast();
                break;
            }
        } else {
            key = atoi(cmd);
            if (f == 1) {
                insert(key);
            } else if (f == 2) {
                delete(key);
            }
        }
    }
     
    printNodes();
     
    release();
 
  return 0;
}

Compiler output:
main.c:9:15: warning: trigraph ??/ ignored, use -trigraphs to enable [-Wtrigraphs]
    9 | }NODE,*PNODE;???//PNODE???malloc???????????????
main.c:9:14: error: expected identifier or '(' before '?' token
    9 | }NODE,*PNODE;???//PNODE???malloc???????????????
      |              ^
main.c: In function 'init':
main.c:14:3: error: 'root' undeclared (first use in this function)
   14 |   root = malloc(sizeof(NODE));
      |   ^~~~
main.c:14:3: note: each undeclared identifier is reported only once for each function it appears in
main.c:14:10: error: implicit declaration of function 'malloc' [-Wimplicit-function-declaration]
   14 |   root = malloc(sizeof(NODE));
      |          ^~~~~~
main.c:4:1: note: include '<stdlib.h>' or provide a declaration of 'malloc'
    3 | #include <string.h>
  +++ |+#include <stdlib.h>
    4 | 
main.c:14:10: warning: incompatible implicit declaration of built-in function 'malloc' [-Wbuiltin-declaration-mismatch]
   14 |   root = malloc(sizeof(NODE));
      |          ^~~~~~
main.c:14:10: note: include '<stdlib.h>' or provide a declaration of 'malloc'
main.c: In function 'insert':
main.c:19:6: error: type of 'key' defaults to 'int' [-Wimplicit-int]
   19 | void insert(key) {
      |      ^~~~~~
main.c:20:15: warning: incompatible implicit declaration of built-in function 'malloc' [-Wbuiltin-declaration-mismatch]
   20 |     PNODE t = malloc(sizeof(NODE));
      |               ^~~~~~
main.c:20:15: note: include '<stdlib.h>' or provide a declaration of 'malloc'
main.c:22:15: error: 'root' undeclared (first use in this function)
   22 |     t->next = root->next;
      |               ^~~~
main.c: In function 'deleteNode':
main.c:29:17: error: 'root' undeclared (first use in this function)
   29 |     if (node == root) {
      |                 ^~~~
main.c:34:5: error: implicit declaration of function 'free' [-Wimplicit-function-declaration]
   34 |     free(node);
      |     ^~~~
main.c:34:5: note: include '<stdlib.h>' or provide a declaration of 'free'
main.c:34:5: warning: incompatible implicit declaration of built-in function 'free' [-Wbuiltin-declaration-mismatch]
main.c:34:5: note: include '<stdlib.h>' or provide a declaration of 'free'
main.c: In function 'delete':
main.c:37:6: error: type of 'key' defaults to 'int' [-Wimplicit-int]
   37 | void delete(key) {
      |      ^~~~~~
main.c:38:17: error: 'root' undeclared (first use in this function)
   38 |     PNODE cur = root->next;
      |                 ^~~~
main.c: In function 'deleteFirst':
main.c:49:16: error: 'root' undeclared (first use in this function)
   49 |     deleteNode(root->next);
      |                ^~~~
main.c: In function 'deleteLast':
main.c:53:16: error: 'root' undeclared (first use in this function)
   53 |     deleteNode(root->prev);
      |                ^~~~
main.c: In function 'printNodes':
main.c:57:17: error: 'root' undeclared (first use in this function)
   57 |     PNODE cur = root->next;
      |                 ^~~~
main.c: In function 'release':
main.c:71:22: error: 'root' undeclared (first use in this function)
   71 |     PNODE tmp, cur = root->next;
      |                      ^~~~
main.c:77:5: warning: incompatible implicit declaration of built-in function 'free' [-Wbuiltin-declaration-mismatch]
   77 |     free(root);
      |     ^~~~
main.c:77:5: note: include '<stdlib.h>' or provide a declaration of 'free'
main.c: In function 'main':
main.c:106:19: error: implicit declaration of function 'atoi' [-Wimplicit-function-declaration]
  106 |             key = atoi(cmd);
      |                   ^~~~