Language: C

void merge(struct Card A[], int n,int left,int mid,int right){
  int i,j,k;
  int n1 = mid -left;
  int n2 = right - mid;
  for(i=0;i<n1;i++)L[i] = A[left + i];
  for(i=0;i<n2;i++)R[i] = A[mid + i];
  L[n1].value = R[n2].value = SENTINEL;
  i=j=0;
    for(k = left; k<right;k++){
      if(L[i].value <=R[j].value){
	A[k]= L[i++];
      }else{
	A[k]= R[j++];
      }
    }
}

void mergeSort(struct Card A[], int n,int left,int right){
  int mid;
  if(left + 1 < right){
    mid = (left + right) /2;
    mergeSort(A,n,left,mid);
    mergeSort(A,n,mid,right);
    merge(A,n,left,mid,right);
  }
}

int partition(struct Card A[], int n, int p,int r){
  int i,j;
  struct Card t,x;
  x = A[r];
  i =p -1;
  for(j = p;j<r;j++){
    if(A[j].value <= x.value){
      i++;
      t=A[i];A[i] =A[j];A[j] =t;
    }
  }
  t=A[i+1];A[i+1] = A[r];A[r]=t;
  return i + 1;
}

void quickSort(struct Card A[],int n,int p,int r){
  int q;
  if(p<r){
    q = partition(A,n,p,r);
    quickSort(A,n,p,q-1);
    quickSort(A,n,q+1,r);
  }
}

int main(){
  int n,i,v;
  struct Card A[MAX],B[MAX];
  char S[10];
  int stable =1;
  
  scanf("%d", &n);
  
  for(i = 0; i < n; i++){
    scanf("%s %d",S,&v);
    A[i].suit = B[i].suit = S[0];
    A[i].value = B[i].value = v;
  }
  
  mergeSort(A,n,0,n);
  quickSort(B,n,0,n-1);
  
  for(i = 0; i < n;i++){
    if(A[i].suit != B[i].suit) stable = 0;
  }
  
  if(stable == 1)printf("stable\n");
  else printf("Not stable\n");
  for(i=0;i<n;i++){
    printf("%c %d\n",B[i].suit,B[i].value);
  }
  
  return 0;
}

Compiler output:
main.c:1:24: error: array type has incomplete element type 'struct Card'
    1 | void merge(struct Card A[], int n,int left,int mid,int right){
      |                        ^
main.c:1:19: warning: 'struct Card' declared inside parameter list will not be visible outside of this definition or declaration
    1 | void merge(struct Card A[], int n,int left,int mid,int right){
      |                   ^~~~
main.c: In function 'merge':
main.c:5:20: error: 'L' undeclared (first use in this function)
    5 |   for(i=0;i<n1;i++)L[i] = A[left + i];
      |                    ^
main.c:5:20: note: each undeclared identifier is reported only once for each function it appears in
main.c:6:20: error: 'R' undeclared (first use in this function)
    6 |   for(i=0;i<n2;i++)R[i] = A[mid + i];
      |                    ^
main.c:7:31: error: 'SENTINEL' undeclared (first use in this function)
    7 |   L[n1].value = R[n2].value = SENTINEL;
      |                               ^~~~~~~~
main.c: At top level:
main.c:18:28: error: array type has incomplete element type 'struct Card'
   18 | void mergeSort(struct Card A[], int n,int left,int right){
      |                            ^
main.c:18:23: warning: 'struct Card' declared inside parameter list will not be visible outside of this definition or declaration
   18 | void mergeSort(struct Card A[], int n,int left,int right){
      |                       ^~~~
main.c: In function 'mergeSort':
main.c:22:15: error: type of formal parameter 1 is incomplete
   22 |     mergeSort(A,n,left,mid);
      |               ^
main.c:23:15: error: type of formal parameter 1 is incomplete
   23 |     mergeSort(A,n,mid,right);
      |               ^
main.c:24:11: error: type of formal parameter 1 is incomplete
   24 |     merge(A,n,left,mid,right);
      |           ^
main.c: At top level:
main.c:28:27: error: array type has incomplete element type 'struct Card'
   28 | int partition(struct Card A[], int n, int p,int r){
      |                           ^
main.c:28:22: warning: 'struct Card' declared inside parameter list will not be visible outside of this definition or declaration
   28 | int partition(struct Card A[], int n, int p,int r){
      |                      ^~~~
main.c: In function 'partition':
main.c:30:15: error: storage size of 't' isn't known
   30 |   struct Card t,x;
      |               ^
main.c:30:17: error: storage size of 'x' isn't known
   30 |   struct Card t,x;
      |                 ^
main.c: At top level:
main.c:43:28: error: array type has incomplete element type 'struct Card'
   43 | void quickSort(struct Card A[],int n,int p,int r){
      |                            ^
main.c:43:23: warning: 'struct Card' declared inside parameter list will not be visible outside of this definition or declaration
   43 | void quickSort(struct Card A[],int n,int p,int r){
      |                       ^~~~
main.c: In function 'quickSort':
main.c:46:19: error: type of formal parameter 1 is incomplete
   46 |     q = partition(A,n,p,r);
      |                   ^
main.c:47:15: error: type of formal parameter 1 is incomplete
   47 |     quickSort(A,n,p,q-1);
      |               ^
main.c:48:15: error: type of formal parameter 1 is incomplete
   48 |     quickSort(A,n,q+1,r);
      |               ^
main.c: In function 'main':
main.c:54:17: error: 'MAX' undeclared (first use in this function)
   54 |   struct Card A[MAX],B[MAX];
      |                 ^~~
main.c:58:3: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   58 |   scanf("%d", &n);
      |   ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | void merge(struct Card A[], int n,int left,int mid,int right){
main.c:58:3: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   58 |   scanf("%d", &n);
      |   ^~~~~
main.c:58:3: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:66:13: error: type of formal parameter 1 is incomplete
   66 |   mergeSort(A,n,0,n);
      |             ^
main.c:67:13: error: type of formal parameter 1 is incomplete
   67 |   quickSort(B,n,0,n-1);
      |             ^
main.c:73:18: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   73 |   if(stable == 1)printf("stable\n");
      |                  ^~~~~~
main.c:73:18: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:73:18: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:73:18: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:74:8: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   74 |   else printf("Not stable\n");
      |        ^~~~~~
main.c:74:8: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:76:5: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   76 |     printf("%c %d\n",B[i].suit,B[i].value);
      |     ^~~~~~
main.c:76:5: note: include '<stdio.h>' or provide a declaration of 'printf'