Find the longest substring within a string that contains at most 2 unique characters.

```
substring("a") => "a"
substring("aaa") => "aaa"
substring("abacd") => "aba"
substring("abacddcd") => "cddcd"
substring("cefageaacceaccacca") => "accacca"
```

This function will take alphanumeric characters as input.

In cases where there could be more than one correct answer, the first string occurrence should be used. For example, substring('abc') should return 'ab' instead of 'bc'.

Although there are O(N^2) solutions to this problem, you should try to solve this problem in O(N) time. Tests may pass for O(N^2) solutions but, this is not guaranteed.

This question is much harder than some of the other substring questions. It's easy to think that you have a solution and then get hung up on the implementation.

Difficulty: EASY

Starter code:
def substring(s):
	

Solutions:
def substring(s):
	(r, rm) = ([], [])
	for (i, x) in enumerate(s):
		if x in r or len(set(r)) < 2:
			r += x
		else:
			if len(r) > len(rm):
				rm = r[:]
			r = [y for y in r[-1::-1] if y == r[-1]] + [x]
	if len(r) > len(rm):
		rm = r[:]
	return ''.join(rm)


def substring(strng):
	best = ''
	best_end = ''
	for c in strng:
		best_end += c
		while len(set(best_end + c)) > 2:
			best_end = best_end[1:]
		if len(best_end) > len(best):
			best = best_end
	return best


from itertools import groupby

def substring(strng):
	return max(enumerate(iter_candidates(strng, 2)), key=lambda ix: (len(ix[1]), -ix[0]))[1]

def iter_candidates(strng, n):
	xs = []
	seen = set()
	for (c, grp) in groupby(strng):
		if len(seen) >= n and c not in seen:
			yield ''.join(xs)
			seen.discard(xs[-n][0])
			del xs[:1 - n]
		xs.append(''.join(grp))
		seen.add(c)
	yield ''.join(xs)


REGEX = __import__('re').compile('((.)\\2*)(?=((.)\\4*(?:\\2|\\4)*))').findall

def substring(strng):
	if len(set(strng)) <= 2:
		return strng
	return max((x + y for (x, _, y, _) in REGEX(strng)), key=len)


def substring(stg):
	(i, mi, s, ms, l) = (0, 0, 3, 2, len(stg))
	while i + s <= l:
		if len(set(stg[i:i + s])) > 2:
			i += 1
		else:
			(mi, ms) = (i, s)
			s += 1
	return stg[mi:mi + ms]


def substring(strng):
	if not strng or len(strng) <= 2:
		return strng
	max_str = ''
	start_ind = 0
	next_ind = 0
	for (ind, letter) in enumerate(strng):
		if strng[start_ind] != letter:
			if start_ind == next_ind:
				next_ind = ind
			elif len(set(strng[start_ind:ind + 1])) > 2:
				(start_ind, next_ind) = (next_ind, ind)
				continue
		if ind - start_ind + 1 > len(max_str):
			max_str = strng[start_ind:ind + 1]
	return max_str


def substring(s, n=2):
	(substr, letters, indexes) = ('', [''] * n, {'': 0})
	for (i, c) in enumerate(s, 1):
		if c not in letters:
			letters = letters[1:] + [c]
			indexes[c] = i - 1
		if i - indexes[letters[0]] > len(substr):
			substr = s[indexes[letters[0]]:i]
	return substr


import re

def substring(s):
	if len(s) < 2:
		return s
	pat = re.compile('(.)\\1*(.)(\\1|\\2)*', re.DOTALL)
	return max((pat.search(s, i).group() for i in range(len(s) - 1)), key=len)


def substring(s):
	r = []
	for (i, x) in enumerate(s):
		temp = [x]
		for j in range(i + 1, len(s)):
			if s[j] in temp or len(set(temp)) < 2:
				temp.append(s[j])
			else:
				r.append(''.join(temp))
				break
		r.append(''.join(temp))
	return s and max(r, key=len) or ''