Longest Substring in Which Every Present Letter Occurs Equally Often

A string is balanced if every letter that occurs in it occurs the same number of times. For example, "aabb", "abc" and "zzz" are balanced, while "aab" is not. Given a string of lowercase English letters, return the length of its longest balanced substring.

Function Signature

def longest_balanced_substring(s: str) -> int:

Rules

• A substring is a contiguous, non-empty part of s.
• Only the letters that occur in the substring are compared; letters that do not occur in it are ignored. A substring made of one repeated letter is therefore balanced.
• Every single character is a balanced substring, so the answer is at least 1.

Constraints

• 1 <= len(s) <= 1000
• s consists only of lowercase English letters a to z.

Examples

Example 1

Input: s = "aabbc"
Output: 4

"aabb" contains a twice and b twice. The only longer substring is the whole string, in which c occurs once while a and b occur twice.

Example 2

Input: s = "aaab"
Output: 3

The whole string has three a and one b, so it is not balanced. "aaa" contains only a, so it is balanced.

Example 3

Input: s = "xyzzyxx"
Output: 6

Both "xyzzyx" and "yzzyxx" contain each of x, y and z exactly twice. The whole string contains x three times, so it is not balanced.