[leetcode] 3. Longest Substring Without Repeating Characters @ python

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原题

https://leetcode.com/problems/longest-substring-without-repeating-characters/
Given a string, find the length of the longest substring without repeating characters.

Example 1:

Input: “abcabcbb”
Output: 3
Explanation: The answer is “abc”, with the length of 3.
Example 2:

Input: “bbbbb”
Output: 1
Explanation: The answer is “b”, with the length of 1.
Example 3:

Input: “pwwkew”
Output: 3
Explanation: 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.

解题思路

遍历字符串, 用字典来储存字符和索引, 如果字符重复了, 更新subtring的起点start, 每次遍历后, 更新result的值. 这里start应该只能增加不能减少, 否则特殊情况时, 例如’abba’ 会出错.

时间复杂度: O(len(s))
空间复杂度: O(1)

代码

class Solution:
    def lengthOfLongestSubstring(self, s):
        """
        :type s: str
        :rtype: int
        """
        # use start to record the start index of substring
        start, res = 0, 0
        seen = {}
        for i, ch in enumerate(s):
            if ch in seen:
                # start cannot decrease
                start = max(start, seen[ch] + 1)
            seen[ch] = i
            res = max(res, i-start+1)
        return res