Language: C

#include <stdio.h>
#define MAX 100000
#define SEN 20000000000

typedef struct{
  char suit;
  int value;
}Card;

struct Card L[MAX/2+2], R[MAX/2+2];

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=SEN;

  i=j=0;

  for(k=left;k<right;k++){
    if(L[i].value<=R[i].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 part(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 quick(struct Card A[], int n, int p, int r){
  int q;
  if(p<r){
    q=part(A,n,p,r);
    quick(A,n,p,q-1);
    quick(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);
  quick(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:10:13: error: array type has incomplete element type 'struct Card'
   10 | struct Card L[MAX/2+2], R[MAX/2+2];
      |             ^
main.c:10:25: error: array type has incomplete element type 'struct Card'
   10 | struct Card L[MAX/2+2], R[MAX/2+2];
      |                         ^
main.c:12:24: error: array type has incomplete element type 'struct Card'
   12 | void merge(struct Card A[], int n, int left, int mid, int right){
      |                        ^
main.c:35:28: error: array type has incomplete element type 'struct Card'
   35 | void mergesort(struct Card A[], int n, int left, int right){
      |                            ^
main.c: In function 'mergesort':
main.c:40:15: error: type of formal parameter 1 is incomplete
   40 |     mergesort(A,n,left,mid);
      |               ^
main.c:41:15: error: type of formal parameter 1 is incomplete
   41 |     mergesort(A,n,mid,right);
      |               ^
main.c:42:11: error: type of formal parameter 1 is incomplete
   42 |     merge(A,n,left,mid,right);
      |           ^
main.c: At top level:
main.c:46:22: error: array type has incomplete element type 'struct Card'
   46 | int part(struct Card A[], int n, int p, int r){
      |                      ^
main.c: In function 'part':
main.c:48:15: error: storage size of 't' isn't known
   48 |   struct Card t, x;
      |               ^
main.c:48:18: error: storage size of 'x' isn't known
   48 |   struct Card t, x;
      |                  ^
main.c: At top level:
main.c:67:24: error: array type has incomplete element type 'struct Card'
   67 | void quick(struct Card A[], int n, int p, int r){
      |                        ^
main.c: In function 'quick':
main.c:70:12: error: type of formal parameter 1 is incomplete
   70 |     q=part(A,n,p,r);
      |            ^
main.c:71:11: error: type of formal parameter 1 is incomplete
   71 |     quick(A,n,p,q-1);
      |           ^
main.c:72:11: error: type of formal parameter 1 is incomplete
   72 |     quick(A,n,q+1,r);
      |           ^
main.c: In function 'main':
main.c:78:15: error: array type has incomplete element type 'struct Card'
   78 |   struct Card A[MAX],B[MAX];
      |               ^
main.c:78:22: error: array type has incomplete element type 'struct Card'
   78 |   struct Card A[MAX],B[MAX];
      |                      ^
main.c:91:13: error: type of formal parameter 1 is incomplete
   91 |   mergesort(A,n,0,n);
      |             ^
main.c:92:9: error: type of formal parameter 1 is incomplete
   92 |   quick(B,n,0,n-1);
      |         ^