LeetCode Longest Substring Without Repeating Characters

class Solution:
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        start=0
        max_length=0
        n=len(s)
        sub_str=''
        for end in range(n):
            if s[end] not in sub_str:
                sub_str+=s[end]
            else:
                max_length=max(max_length,len(sub_str))
                while s[start]!=s[end]:
                    start+=1
                start+=1
                sub_str=s[start:end+1]
        return max(max_length,len(sub_str))

class Solution:
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        chars = {}
        begin = 0
        result = 0
        for i, c in enumerate(s, 1):
            if (c in chars) and (chars[c] > begin):
                begin = chars[c]
            elif (i - begin > result):
                result = i - begin              
            chars[c] = i
        return result