# Task
 Given an array `arr` and a number `n`. Call a pair of numbers from the array a `Perfect Pair` if their sum is equal to `n`.

 Find all of the perfect pairs and return the sum of their **indices**. 
 
 Note that any element of the array can only be counted in one Perfect Pair. Also if there are multiple correct answers, return the smallest one.

# Example

 For `arr = [1, 4, 2, 3, 0, 5] and n = 7`, the result should be `11`.
 
 Since the Perfect Pairs are `(4, 3)` and `(2, 5)` with indices `1 + 3 + 2 + 5 = 11`.

 For `arr = [1, 3, 2, 4] and n = 4`, the result should be `1`.
 
 Since the element at `index 0` (i.e. 1) and the element at `index 1` (i.e. 3) form the only Perfect Pair.

# Input/Output


 - `[input]` integer array `arr`

  array of non-negative integers.


 - `[input]` integer `n`

  positive integer


 - `[output]` integer

  sum of indices and 0 if no Perfect Pair exists.

Difficulty: EASY
Starter code: def pairwise(arr, n):
	

Solutions:
def pairwise(arr, n):
	s = []
	for i in range(len(arr) - 1):
		for j in range(i + 1, len(arr)):
			if j in s or i in s:
				continue
			if arr[i] + arr[j] == n:
				s.append(i)
				s.append(j)
	return sum(s)


def pairwise(arr, n):
	(iMap, s) = ({}, 0)
	for (i, x) in enumerate(arr):
		iMap[x] = iMap.get(x, []) + [i]
	for x in arr:
		if n - x in iMap and len(iMap[n - x]) > (x == n - x) < len(iMap[x]):
			s += iMap[x].pop(0)
			s += iMap[n - x].pop(0)
	return s


def pairwise(arr, n):
	seen = set()
	for i in range(len(arr)):
		for j in range(i + 1, len(arr)):
			if arr[i] + arr[j] == n and (not (i in seen or j in seen)):
				seen.add(i)
				seen.add(j)
				break
	return sum(seen)


def pairwise(arr, n):
	(a, i, j, pairs) = (sorted(arr), 0, len(arr) - 1, [])
	while i < j:
		tot = a[i] + a[j]
		if tot == n:
			pairs.extend([a[i], a[j]])
			(i, j) = (i + 1, j - 1)
		elif tot < n:
			i += 1
		else:
			j -= 1
	pairs.sort()
	indices = []
	(i, last) = (0, None)
	for x in pairs:
		i = indices[-1] + 1 if x == last else 0
		indices.append(arr.index(x, i))
		last = x
	return sum(indices)


def pairwise(arr, n):
	save = set()
	add = save.add
	return sum((i + j if x + y == n and (not (i in save or add(j) or add(i))) else 0 for (i, x) in enumerate(arr) if i not in save for (j, y) in enumerate(arr[i + 1:], i + 1) if j not in save))


def pairwise(arr, n):
	result = 0
	for i in range(len(arr)):
		d = n - arr[i]
		if d in arr[i + 1:]:
			j = arr.index(d, i + 1)
			result += i + j
			arr[i] = arr[j] = n + 1
	return result


from collections import defaultdict

def pairwise(arr, n):
	ixs = defaultdict(list)
	for (i, e) in enumerate(arr):
		ixs[e].append(i)
	h = n / 2
	t = sum((x + y for (a, ai) in (p for p in ixs.items() if p[0] < h) for (x, y) in zip(ai, ixs.get(n - a, []))))
	if n % 2:
		return t
	hi = ixs.get(int(h), [])
	return t + sum(hi[:len(hi) // 2 * 2])


def pairwise(arr, n):
	used = []
	for i in range(len(arr) - 1):
		for j in range(i + 1, len(arr)):
			if arr[i] + arr[j] == n and (not (i in used or j in used)):
				used.extend([i, j])
	return sum(used)


from itertools import combinations

def pairwise(arr, n):
	(r, c) = (0, [])
	for ((i, a), (j, b)) in combinations(enumerate(arr), 2):
		if a + b == n and i not in c and (j not in c):
			r += i + j
			c.extend([i, j])
	return r


def pairwise(arr, n):
	lst = [[e, i] for (i, e) in enumerate(arr)]
	(ans, v) = ([], [])
	for (i, e) in enumerate(arr):
		if i in v:
			continue
		for (j, e2) in enumerate(arr[i + 1:]):
			x = i + 1 + j
			if e + e2 == n and x not in v:
				v.append(i)
				v.append(x)
				ans.append((i, x))
				break
	return sum([i[0] + i[1] for i in ans]) if ans != [] else 0