Problem:
Quick Sort 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). Input The first line contains an integer n , the number of cards.  n cards are given in the following lines. Each card is given in a line and represented by a pair of a character and an integer separated by a single space. Output In the first line, print the stability (" Stable " or " Not stable ") of this output. In the following lines, print the arranged cards in the same manner of that of the input. Constraints  1 ≤ n ≤ 100,000  1 ≤ the number of a card  ≤ 10 9  There are no identical card in the input  Sample Input 1 6
D 3
H 2
D 1
S 3
D 2
C 1 Sample Output 1 Not stable
D 1
C 1
D 2
H 2
D 3
S 3 Sample Input 2 2
S 1
H 1 Sample Output 2 Stable
S 1
H 1

Solution (Python):
class Card:
    def __init__(self, suit, value):
        self.suit = suit
        self.value = value


def merge(a, left, mid, right):
    n1 = mid - left
    n2 = right - mid
    l = a[left:left+n1]
    r = a[mid:mid+n2]
    l.append(Card('X', float('inf')))
    r.append(Card('X', float('inf')))
    i = 0
    j = 0
    for k in range(left, right):
        if l[i].value <= r[j].value:
            a[k] = l[i]
            i += 1
        else:
            a[k] = r[j]
            j += 1


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


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


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


n = int(input())
a = []
b = []
for i in range(n):
    tmp = input().split()
    a.append(Card(tmp[0], int(tmp[1])))
    b.append(Card(tmp[0], int(tmp[1])))

mergeSort(a, 0, n)
quickSort(b, 0, n-1)

is_stable = True
for i in range(n):
    if a[i].suit != b[i].suit:
        is_stable = False
        break

if is_stable:
    print('Stable')
else:
    print('Not stable')

for v in b:
    print('%s %d' % (v.suit, v.value))

Solution (Python):

class Card():
    def __init__(self,suit,num):
        """suit and num are str"""
        self.s = suit
        self.n = int(num)
    def __str__(self):
        return self.s + ' ' + str(self.n)

def partition(A,p,r):
    x = A[r].n
    i = p - 1
    for j in range(p,r,1):
        if A[j].n <= x:
            i += 1
            tmp = A[i]
            A[i] = A[j]
            A[j] = tmp
    tmp = A[i+1]
    A[i+1] = A[r]
    A[r] = tmp
    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)
def is_stable(Org,A):
    cop = A[:]
    for org in Org:
        i = cop.index(org)
        if i > 0:
            if A[i - 1].n == org.n:
                return False
        del cop[i]
    return True
n = int(input())
A = [Card(*input().split()) for _ in range(n)]
Or = A[:]
quick_sort(A,0,len(A)-1)
if is_stable(Or,A): print('Stable')
else: print('Not stable')
for a in A: print(a)