3. Longest Substring Without Repeating Characters

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本文介绍了一种解决LeetCode上经典问题“最长无重复字符子串”的算法。该问题要求找出给定字符串中无重复字符的最长子串长度。通过使用字典记录字符位置并迭代更新子串起始位置，实现高效求解。

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https://leetcode.com/problems/longest-substring-without-repeating-characters/description/

Given a string, find the length of the longest substring without repeating characters.

Given  "abcabcbb", the answer is "abc", which the length is 3.
Given "bbbbb", the answer is "b", with the length of 1.
Given "pwwkew", the answer is "wke", with the length of 3. 
Note that the answer must be a substring, "pwke" is a subsequence and not a substring.

To make it clear, the return type is an integer, which is the length of the longest substring, so we do not need to save the substring itself. One easy way is using a dictionary to save every character and their position in the original string, using an integer to save the start position of current substring.

class Solution(object):
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        char_dict = {}
        max_length = 0
        length = 0
        start_index = 0
        for i in range(len(s)):
            if s[i] in char_dict and char_dict[s[i]] >= start_index:
                start_index = char_dict[s[i]] + 1
                char_dict[s[i]] = i
                max_length = max(max_length, length)
                length = i - start_index + 1
            else:
                char_dict[s[i]] = i
                length += 1
        return max(length, max_length)