substring_two_chars.py

'''
Question statement:
Return the longest contiguous substring of 2 distinct characters from an input string.
Example
input: abbaacab
output : abbaa

input: abcefabbabaabefghghfa 
return: abbabaab

input: aabceddddcdccecabceftg 
return: ddddcdcc

input: acbabbcbca 
return : bbcbc
'''


def max_contiguous(string):
	if len(string) <= 2:
		return string

	curr_max_str = curr_blob = string[:2]
	char0, char1 = string[0], string[1]
	last_char = char1
	if char0 == char1:
		last_contiguous_idx = 0
		last_last_char = None
	else:
		last_contiguous_idx = 1
		last_last_char = char0

	for i, char in enumerate(string[2:]):
		if last_last_char is None:
			curr_blob += char

			if char != last_char:
				last_contiguous_idx = i + 2
				last_last_char = last_char
				last_char = char
		else:
			if char == last_char:
				curr_blob += char
			elif char == last_last_char:
				curr_blob += char
				last_contiguous_idx = i + 2
				last_last_char = last_char
				last_char = char
			else:
				curr_blob = string[last_contiguous_idx : i + 3]
				last_contiguous_idx = i + 2
				last_last_char = last_char
				last_char = char

		if len(curr_blob) > len(curr_max_str):
			curr_max_str = curr_blob[:]

	return curr_max_str

assert max_contiguous('abbaacab') == 'abbaa'
assert max_contiguous('abcefabbabaabefghghfa') == 'abbabaab'
assert max_contiguous('aabceddddcdccecabceftg') == 'ddddcdcc'
assert max_contiguous('acbabbcbca') == 'bbcbc'
assert max_contiguous('') == ''
assert max_contiguous('aaaaaaaa') == 'aaaaaaaa'
assert max_contiguous('aaaaabbbbb3dasfa938209320202020202020202') == '20202020202020202'
assert max_contiguous('aaaaabbbbb3dasfa938209320202020202020202afdafsfasdweeweeeeee') == '20202020202020202'