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):
#coding:utf-8
#1_6_C
def partition(A, p, r):
    x = int(A[r][1])
    i = p - 1
    for j in range(p, r):
        if int(A[j][1]) <= x:
            i += 1
            A[i], A[j] = A[j], A[i]
    A[i + 1], A[r] = A[r], A[i + 1]
    return i + 1

def quick_sort(A, p, r):
    if p < r:
        q = partition(A, p, r)
        quick_sort(A, p, q - 1)
        quick_sort(A, q + 1, r)
    return A

class CheckStable:
    def __init__(self):
        self.d = dict() # Key is number. Value is array of suit.

    def store_cards(self, cards):
        for i in range(n):
            if cards[i][1] in self.d:
                self.d[cards[i][1]].append(cards[i][0])
            else:
                self.d[cards[i][1]] = [cards[i][0]]

    def is_stable(self, cards):
        for item in reversed(cards):
            if self.d[item[1]].pop() != item[0]:
                return "Not Stable"
        return "Stable"

n = int(input())
cards = [tuple(input().split()) for i in range(n)]
d = CheckStable()
d.store_cards(cards)

quick_sort(cards, 0, n - 1)

print(d.is_stable(cards))
for i in range(n):
    print(' '.join(cards[i]))

Fixed, accepted submission:
#coding:utf-8
#1_6_C
def partition(A, p, r):
    x = int(A[r][1])
    i = p - 1
    for j in range(p, r):
        if int(A[j][1]) <= x:
            i += 1
            A[i], A[j] = A[j], A[i]
    A[i + 1], A[r] = A[r], A[i + 1]
    return i + 1

def quick_sort(A, p, r):
    if p < r:
        q = partition(A, p, r)
        quick_sort(A, p, q - 1)
        quick_sort(A, q + 1, r)
    return A

class CheckStable:
    def __init__(self):
        self.d = dict() # Key is number. Value is array of suit.

    def store_cards(self, cards):
        for i in range(n):
            if cards[i][1] in self.d:
                self.d[cards[i][1]].append(cards[i][0])
            else:
                self.d[cards[i][1]] = [cards[i][0]]

    def is_stable(self, cards):
        for item in reversed(cards):
            if self.d[item[1]].pop() != item[0]:
                return "Not stable"
        return "Stable"

n = int(input())
cards = [tuple(input().split()) for i in range(n)]
d = CheckStable()
d.store_cards(cards)

quick_sort(cards, 0, n - 1)

print(d.is_stable(cards))
for i in range(n):
    print(' '.join(cards[i]))