Problem p02277 (C++)

Buggy submission (status: Runtime Error):
#include <iostream>
#include <stdio.h>

#define SENTINEL 1000000001

struct Card {
  char suit;
  int value;
};

void trace(Card a[], int size) {
  for (int i = 0; i < size; ++i) {
    printf("%c %d\n", a[i].suit, a[i].value);
  }
}

void Merge(Card A[], int left, int mid, int right) {
  int n1 = mid - left;
  int n2 = right - mid;
  Card L[n1 + 1], R[n2 + 1];
  for (int i = 0; i < n1; ++i) {
    L[i] = A[left + i];
  }
  for (int i = 0; i < n2; ++i) {
    R[i] = A[mid + i];
  }
  L[n1].value = SENTINEL;
  R[n2].value = SENTINEL;
  int m = 0;
  int n = 0;
  for (int i = left; i < right; ++i) {
    if (L[m].value <= R[n].value) {
      A[i] = L[m];
      ++m;
    } else {
      A[i] = R[n];
      ++n;
    }
  }
}

void MergeSort(Card A[], int left, int right) {
  if (left >= right - 1)
    return;
  int mid = (left + right) / 2;
  MergeSort(A, left, mid);
  MergeSort(A, mid, right);
  Merge(A, left, mid, right);
}

int Partition(Card A[], int p, int r) {
  int x = A[r].value;
  int j = p;
  for (int i = p; i < r; ++i) {
    if (A[i].value <= x) {
      std::swap(A[j], A[i]);
      ++j;
    }
  }
  std::swap(A[j], A[r]);
  return j;
}

void QuickSort(Card A[], int p, int r) {
  if (p >= r)
    return;
  int q = Partition(A, p, r);
  QuickSort(A, p, q - 1);
  QuickSort(A, q + 1, r);
}

bool IsStable(Card A[], Card B[], int N) {
  for (int i = 0; i < N; ++i) {
    if (A[i].suit != B[i].suit || A[i].value != B[i].value) {
      return false;
    }
  }
  return true;
}

int main() {
  int N;
  Card A[100], B[100];

  scanf("%d", &N);
  for (int i = 0; i < N; i++) {
    std::cin >> A[i].suit >> A[i].value;
    B[i] = A[i];
  }

  MergeSort(A, 0, N);
  QuickSort(B, 0, N - 1);
  IsStable(A, B, N) ? printf("Stable\n") : printf("Not stable\n");
  trace(B, N);
  return 0;
}

Fixed submission:
#include <iostream>
#include <stdio.h>

#define SENTINEL 1000000001

struct Card {
  char suit;
  int value;
};

void trace(Card a[], int size) {
  for (int i = 0; i < size; ++i) {
    printf("%c %d\n", a[i].suit, a[i].value);
  }
}

void Merge(Card A[], int left, int mid, int right) {
  int n1 = mid - left;
  int n2 = right - mid;
  Card L[n1 + 1], R[n2 + 1];
  for (int i = 0; i < n1; ++i) {
    L[i] = A[left + i];
  }
  for (int i = 0; i < n2; ++i) {
    R[i] = A[mid + i];
  }
  L[n1].value = SENTINEL;
  R[n2].value = SENTINEL;
  int m = 0;
  int n = 0;
  for (int i = left; i < right; ++i) {
    if (L[m].value <= R[n].value) {
      A[i] = L[m];
      ++m;
    } else {
      A[i] = R[n];
      ++n;
    }
  }
}

void MergeSort(Card A[], int left, int right) {
  if (left >= right - 1)
    return;
  int mid = (left + right) / 2;
  MergeSort(A, left, mid);
  MergeSort(A, mid, right);
  Merge(A, left, mid, right);
}

int Partition(Card A[], int p, int r) {
  int x = A[r].value;
  int j = p;
  for (int i = p; i < r; ++i) {
    if (A[i].value <= x) {
      std::swap(A[j], A[i]);
      ++j;
    }
  }
  std::swap(A[j], A[r]);
  return j;
}

void QuickSort(Card A[], int p, int r) {
  if (p >= r)
    return;
  int q = Partition(A, p, r);
  QuickSort(A, p, q - 1);
  QuickSort(A, q + 1, r);
}

bool IsStable(Card A[], Card B[], int N) {
  for (int i = 0; i < N; ++i) {
    if (A[i].suit != B[i].suit || A[i].value != B[i].value) {
      return false;
    }
  }
  return true;
}

int main() {
  int N;
  Card A[100000], B[100000];

  scanf("%d", &N);
  for (int i = 0; i < N; i++) {
    std::cin >> A[i].suit >> A[i].value;
    B[i] = A[i];
  }

  MergeSort(A, 0, N);
  QuickSort(B, 0, N - 1);
  IsStable(A, B, N) ? printf("Stable\n") : printf("Not stable\n");
  trace(B, N);
  return 0;
}