Problem:
Let's arrange a deck of cards. Your task is to sort totally n cards. A card consists of a part of a suit (S, H, C or D) and an number. Write a program which sorts such cards based on the following pseudocode:                 Partition(A, p, r) 1 x = A[r] 2 i = p-1 3 for j = p to r-1 4     do if A[j] <= x 5        then i = i+1 6            exchange A[i] and A[j]  7 exchange A[i+1] and A[r] 8 return i+1   Quicksort(A, p, r) 1 if p < r 2    then q = Partition(A, p, r) 3        run Quicksort(A, p, q-1) 4        run Quicksort(A, q+1, r)   Here, A is an array which represents a deck of cards and comparison operations are performed based on the numbers.  Your program should also report the stability of the output for the given input (instance). Here, 'stability of the output' means that: cards with the same value appear in the output in the same order as they do in the input (instance).

Incorrect submission (verdict: Wrong Answer):
def InputData():
    
    sequence_len = int(input())
    sequence = [input().split(" ") for _ in range(sequence_len)]
    sequence_m = [(a[0], int(a[1])) for a in sequence]
    sequence_q = [(a[0], int(a[1])) for a in sequence]
    return sequence_len, sequence_m, sequence_q

def partition(sequence, p, r):
    x = sequence[r]
    i = p - 1
    for j in range(p, r):
        if sequence[j][1] <= x[1]:
            i += 1
            sequence[i], sequence[j]= sequence[j], sequence[i]
    
    sequence[i+1], sequence[r] = sequence[r], sequence[i+1]
    return i+1

def quickSort(sequence, p, r):
    if p < r:
        q = partition(sequence, p, r)
        quickSort(sequence, p, q-1)
        quickSort(sequence, q+1, r)

def merge(sequence, left, mid, right):
    n1 = mid - left
    n2 = right - mid
    sequence_left = sequence[left:left + n1]
    sequence_right = sequence[mid:mid + n2]
    sequence_left.append([None, float('inf')])
    sequence_right.append([None, float('inf')])

    left_cnt = 0
    right_cnt = 0
    for i in range(left, right):
        if sequence_left[left_cnt][1] <= sequence_right[right_cnt][1]:
            sequence[i] = sequence_left[left_cnt]
            left_cnt += 1
        else:
            sequence[i] = sequence_right[right_cnt]
            right_cnt += 1

def mergeSort(sequence, left, right):
    if left + 1 < right:
        mid = (left + right) // 2
        mergeSort(sequence, left, mid)
        mergeSort(sequence, mid, right)
        merge(sequence, left, mid, right)

def PrintOut(sequence_m, sequence_q):
    if sequence_m == sequence_q:
        print("Stable")
    else:
        print("Not Stable")
    
    for a in sequence_q:
        print(a[0], a[1])

def main():
    [sequence_len, sequence_m, sequence_q] = InputData()
    mergeSort(sequence_m, 0, sequence_len)
    quickSort(sequence_q, 0, sequence_len-1)
    
    PrintOut(sequence_m, sequence_q)

if __name__=="__main__":
    main()


Fixed, accepted submission:
def InputData():
    
    sequence_len = int(input())
    sequence = [input().split(" ") for _ in range(sequence_len)]
    sequence_m = [(a[0], int(a[1])) for a in sequence]
    sequence_q = [(a[0], int(a[1])) for a in sequence]
    return sequence_len, sequence_m, sequence_q

def partition(sequence, p, r):
    x = sequence[r]
    i = p - 1
    for j in range(p, r):
        if sequence[j][1] <= x[1]:
            i += 1
            sequence[i], sequence[j]= sequence[j], sequence[i]
    
    sequence[i+1], sequence[r] = sequence[r], sequence[i+1]
    return i+1

def quickSort(sequence, p, r):
    if p < r:
        q = partition(sequence, p, r)
        quickSort(sequence, p, q-1)
        quickSort(sequence, q+1, r)

def merge(sequence, left, mid, right):
    n1 = mid - left
    n2 = right - mid
    sequence_left = sequence[left:left + n1]
    sequence_right = sequence[mid:mid + n2]
    sequence_left.append([None, float('inf')])
    sequence_right.append([None, float('inf')])

    left_cnt = 0
    right_cnt = 0
    for i in range(left, right):
        if sequence_left[left_cnt][1] <= sequence_right[right_cnt][1]:
            sequence[i] = sequence_left[left_cnt]
            left_cnt += 1
        else:
            sequence[i] = sequence_right[right_cnt]
            right_cnt += 1

def mergeSort(sequence, left, right):
    if left + 1 < right:
        mid = (left + right) // 2
        mergeSort(sequence, left, mid)
        mergeSort(sequence, mid, right)
        merge(sequence, left, mid, right)

def PrintOut(sequence_m, sequence_q):
    if sequence_m == sequence_q:
        print("Stable")
    else:
        print("Not stable")
    
    for a in sequence_q:
        print(a[0], a[1])

def main():
    [sequence_len, sequence_m, sequence_q] = InputData()
    mergeSort(sequence_m, 0, sequence_len)
    quickSort(sequence_q, 0, sequence_len-1)
    
    PrintOut(sequence_m, sequence_q)

if __name__=="__main__":
    main()