Leetcode 159. Longest Substring with At Most Two Distinct CharactersLeetcode

Given a string s, return the length of the longest 

substring that contains at most two distinct characters.

Example 1:

Input: s = "eceba"
Output: 3
Explanation: The substring is "ece" which its length is 3.

Example 2:

Input: s = "ccaabbb"
Output: 5
Explanation: The substring is "aabbb" which its length is 5.

Constraints:

• 1 <= s.length <= 105
• s consists of English letters.

I pass the problem in my first try, but my solution is not smart enough. I remember the first character and the second character last position then do some math. Therefore, I can know the length difference. Only need to memorize the longest one.

class Solution:
    def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
        if len(s) == 1:
            return 1

        table = {}

        longest = 0

        c_length = 0

        for i in range(0, len(s)):
            keys = list(table.keys())
            
            if s[i] in keys or len(keys) < 2:
                table[s[i]] = i
                c_length += 1
            else:
                longest = c_length if c_length > longest else longest

                f_i = table[keys[0]]
                s_i = table[keys[1]]

                if f_i > s_i:
                    del table[keys[1]]
                else:
                    del table[keys[0]]
                
                c_length = abs(f_i - s_i) + 1

                table[s[i]] = i
        
        return c_length if c_length > longest else longest

However, I saw there is a brilliant solution. Much cleaner and simpler, but need some time to think about it.

class Solution:
    def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
        count, i = {}, 0 
        for j, k in enumerate(s):
            count[k] = count.get(k, 0) + 1
            if len(count) > 2:
                count[s[i]] -= 1
                if count[s[i]] == 0:
                    del count[s[i]]
                i += 1
        return j - i + 1