3. Longest Substring Without Repeating Characters

class Solution(object):
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        usedchar = {}  #存储无重复得字符
        max_length = start = 0  #初始化最长字符和
        
        for i, c in enumerate(s):
            if c in usedchar and start <= usedchar[c]: #如果有字符在字典中，并且大于子串起始部分，则从新定位子串的起始位置
                start = usedchar[c] + 1
            else:
                max_length = max(max_length, i - start + 1)   #取最大值
                
            usedchar[c] = i
        
        return max_length