# Task
 Sorting is one of the most basic computational devices used in Computer Science. 
 
 Given a sequence (length ≤ 1000) of 3 different key values (7, 8, 9), your task is to find the minimum number of exchange operations necessary to make the sequence sorted. 

 One operation is the switching of 2 key values in the sequence.

# Example

 For `sequence = [7, 7, 8, 8, 9, 9]`, the result should be `0`.
 
 It's already a sorted sequence.

 For `sequence = [9, 7, 8, 8, 9, 7]`, the result should be `1`.
 
 We can switching `sequence[0]` and `sequence[5]`.
 
 For `sequence = [8, 8, 7, 9, 9, 9, 8, 9, 7]`, the result should be `4`.
 
 We can:
```
 [8, 8, 7, 9, 9, 9, 8, 9, 7] 
 switching sequence[0] and sequence[3]
 --> [9, 8, 7, 8, 9, 9, 8, 9, 7]
 switching sequence[0] and sequence[8]
 --> [7, 8, 7, 8, 9, 9, 8, 9, 9]
 switching sequence[1] and sequence[2]
 --> [7, 7, 8, 8, 9, 9, 8, 9, 9]
 switching sequence[5] and sequence[7]
 --> [7, 7, 8, 8, 8, 9, 9, 9, 9] 
```
So `4` is the minimum number of operations for the sequence to become sorted.

# Input/Output


 - `[input]` integer array `sequence`

  The Sequence.


 - `[output]` an integer

  the minimum number of operations.

Difficulty: MEDIUM
Tags: Sorting
Skill types: Sorting

Starter code:
def exchange_sort(sequence):
	

Solutions:
from collections import Counter

def exchange_sort(sequence):
	(swaps, cnt) = (0, Counter())
	for (a, b) in zip(sequence, sorted(sequence)):
		if cnt[b, a] > 0:
			cnt[b, a] -= 1
			swaps += 1
		elif a != b:
			cnt[a, b] += 1
	return swaps + sum(cnt.values()) // 3 * 2


from collections import Counter

def exchange_sort(s):
	(a, b, c) = (Counter(zip(*p)) for p in ((s, sorted(s)), (sorted(s), s), (s, s)))
	return sum(((a | b) + a + b - c - c - c).values()) // 6


def exchange_sort(l):
	(a, b, c) = (l.count(7), -l.count(9), -l.count(8))
	if b != 0:
		return a - l[:a].count(7) + max(l[a:b].count(9), l[b:].count(8))
	t = sorted(l)
	r = 0
	for i in range(len(l)):
		r += 1 if t[i] > l[i] else 0
	return r


from collections import Counter

def exchange_sort(sequence):
	(a, b, c) = map(Counter(sequence).get, (7, 8, 9))
	if not (a and b and c):
		return sum((x < y for (x, y) in zip(sorted(sequence), sequence)))
	return sequence[a:].count(7) + max(sequence[-c:].count(8), sequence[a:-c].count(9))


def exchange_sort(xs):
	(xs7, xs9) = (xs[:xs.count(7)], xs[len(xs) - xs.count(9):])
	return xs7.count(8) + xs9.count(8) + max(xs7.count(9), xs9.count(7))


def exchange_sort(s):
	(t7, t8) = (s.count(7), s.count(8))
	(n97, n79) = (s[:t7].count(9), s[t7 + t8:].count(7))
	(n87, n98) = (s[:t7].count(8), s[t7:t7 + t8].count(9))
	return 2 * n97 + n87 + n98 - min(n79, n97)


def lsorter2(l):
	intsec = [(a, b) for (a, b) in zip(l, sorted(l)) if a != b]
	return list(zip(*intsec))

def naturalSwap(l):
	x1 = lsorter2(l)
	ori = list(x1[0])
	sort = list(x1[1])
	si = [i for (i, x) in enumerate(sort) if x == ori[0]]
	fi = [i for (i, x) in enumerate(ori) if x == sort[0]]
	ci = [x for x in si if x in fi]
	new_ori = [x for (i, x) in enumerate(ori) if i not in ci]
	swap_num1 = len(ci)
	si2 = [i for (i, x) in enumerate(ori) if x == ori[0]]
	fi2 = [i for (i, x) in enumerate(sort) if x == sort[0]]
	ci2 = [x for x in si2 if x in fi2]
	new_ori2 = [x for (i, x) in enumerate(ori) if i not in ci2]
	swap_num2 = len(ci2)
	swap_real = min(swap_num1, swap_num2)
	to_force_swap1 = ci[:swap_real]
	to_force_swap2 = ci2[:swap_real]
	to_force_swap3 = to_force_swap1 + to_force_swap2
	new_list = [x for (i, x) in enumerate(ori) if i not in to_force_swap3]
	return (new_list, swap_real)

def forcedSwap(l):
	x1 = lsorter2(l)
	ori = list(x1[0])
	sort = list(x1[1])
	idx = ori.index(sort[0])
	(ori[0], ori[idx]) = (ori[idx], ori[0])
	return ori

def exchange_sort(l):
	swaps = 0
	dlist = l[:]
	cnt = 1
	if dlist == sorted(dlist):
		return 0
	else:
		while cnt > 0 and len(dlist) > 0:
			cnt = 0
			x1 = naturalSwap(dlist)
			cnt = x1[1]
			swaps += x1[1]
			dlist = x1[0]
		while len(dlist) > 2:
			x2 = forcedSwap(dlist)
			swaps += 1
			dlist = x2
		return swaps


from collections import defaultdict, Counter

def exchange_sort(sequence):
	(n_sevens, n_eights) = (sequence.count(7), sequence.count(8))
	sevens = defaultdict(lambda : 0, Counter(sequence[:n_sevens]))
	eights = defaultdict(lambda : 0, Counter(sequence[n_sevens:n_sevens + n_eights]))
	nines = defaultdict(lambda : 0, Counter(sequence[n_sevens + n_eights:]))
	direct_78 = min(sevens[8], eights[7])
	direct_79 = min(sevens[9], nines[7])
	direct_89 = min(eights[9], nines[8])
	sevens[8] -= direct_78
	nines[7] -= direct_79
	nines[8] -= direct_89
	eights[7] -= direct_78
	eights[9] -= direct_89
	(sevens[7], eights[8], nines[9]) = (0, 0, 0)
	return direct_78 + direct_79 + direct_89 + sum(nines.values()) * 2


def exchange_sort(sequence):
	a = sequence[:]
	seven = a.count(7)
	eight = a.count(8)
	r = 0
	for i in range(seven, len(a)):
		if a[i] == 7:
			r += 1
			if i < seven + eight and 8 in a[:seven]:
				j = a.index(8)
			else:
				for j in range(seven):
					if a[j] != 7:
						break
			(a[i], a[j]) = (a[j], a[i])
	r += a[seven + eight:].count(8)
	return r


def exchange_sort(sequence):
	print(sequence)
	counts = [sequence.count(n) for n in [7, 8, 9]]
	dividers = [0] + [sum(counts[:i]) for i in range(1, 4)]
	groupings = [[sequence[dividers[i]:dividers[i + 1]].count(n) for n in [7, 8, 9]] for i in range(3)]
	n = 0

	def swap(t0, t1, n0, n1):
		swappable = min(groupings[t0][n1], groupings[t1][n0])
		groupings[t0][n0] += swappable
		groupings[t0][n1] -= swappable
		groupings[t1][n1] += swappable
		groupings[t1][n0] -= swappable
		return swappable
	for (a, b) in [(0, 1), (0, 2), (1, 2)]:
		n += swap(a, b, a, b)
	for (a, b, c) in [(0, 1, 2), (1, 2, 0), (2, 0, 1), (2, 1, 0), (1, 0, 2), (0, 2, 1)]:
		n += swap(a, b, a, c)
	for (a, b) in [(0, 1), (0, 2), (1, 2)]:
		n += swap(a, b, a, b)
	return n