Longest Alternating Substring

Given a string of digits, return the longest substring with alternating odd/even or even/odd digits. If two or more substrings have the same length, return the substring that occurs first.

Examples

longest_substring("225424272163254474441338664823") ➞ "272163254"
# substrings = 254, 272163254, 474, 41, 38, 23

longest_substring("594127169973391692147228678476") ➞ "16921472"
# substrings = 94127, 169, 16921472, 678, 476

longest_substring("721449827599186159274227324466") ➞ "7214"
# substrings = 7214, 498, 27, 18, 61, 9274, 27, 32
# 7214 and 9274 have same length, but 7214 occurs first.

Notes

The minimum alternating substring size is 2, and there will always be at least one alternating substring.