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):
class Num:
    a=0
    b=0
    origin=0

def partition(A, p, r):
    x=A[r].b
    i=p-1
    for j in range(p,r):
        if A[j].b <= x:
            i += 1
            A[i].b,A[j].b = A[j].b,A[i].b
            A[i].a,A[j].a = A[j].a,A[i].a
    A[i+1].b,A[r].b = A[r].b,A[i+1].b
    A[i+1].a,A[r].a = A[r].a,A[i+1].a
    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 isStable(A):
    n=len(A)
    for i in range(n-1):
        if A[i].b == A[i+1].b and A[i].origin < A[i+1].origin:
            return False
    return True

if __name__ == "__main__":
    
    n=int(input())
    A=[]
    for i in range(n):
        num =Num()
        a=input().split()
        num.a = a[0]
        num.b = int(a[1])
        num.origin=i
        A.append(num)
    quick_sort(A,0,n-1)
    for i in A:
        print("a={},b={},origin={}".format(i.a,i.b,i.origin))
    if isStable(A):
        print("Stable")
    else:
        print("Not stable")
    for i in A:
        print(i.a,end=" ")
        print(i.b)
            
    


Fixed, accepted submission:
class Num:
    a=0
    b=0
    origin=0

def partition(A, p, r):
    x=A[r].b
    i=p-1
    for j in range(p,r):
        if A[j].b <= x:
            i += 1
            A[i].b,A[j].b = A[j].b,A[i].b
            A[i].a,A[j].a = A[j].a,A[i].a
            A[i].origin,A[j].origin = A[j].origin,A[i].origin
    A[i+1].b,A[r].b = A[r].b,A[i+1].b
    A[i+1].a,A[r].a = A[r].a,A[i+1].a
    A[i+1].origin,A[r].origin = A[r].origin,A[i+1].origin
    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 isStable(A):
    flag=True
    n=len(A)
    for i in range(n-1):
        if A[i].b == A[i+1].b and A[i].origin > A[i+1].origin:
            flag = False
    return flag

if __name__ == "__main__":
    
    n=int(input())
    A=[]
    for i in range(n):
        num =Num()
        a=input().split()
        num.a = a[0]
        num.b = int(a[1])
        num.origin=i
        A.append(num)
    quick_sort(A,0,n-1)
    if isStable(A):
        print("Stable")
    else:
        print("Not stable")
    for i in A:
        print(i.a,end=" ")
        print(i.b)