[leetcode][python] 424. Longest Repeating Character Replacement

424. Longest Repeating Character Replacement

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知识点： two pointers | sliding window

1. 原题

Given a string s that consists of only uppercase English letters, you can perform at most k operations on that string.

In one operation, you can choose any character of the string and change it to any other uppercase English character.

Find the length of the longest sub-string containing all repeating letters you can get after performing the above operations.

Note:
Both the string’s length and k will not exceed 104.

Example 1:

Input:
s = “ABAB”, k = 2

Output:
4

Explanation:
Replace the two 'A’s with two 'B’s or vice versa.

Example 2:

Input:
s = “AABABBA”, k = 1

Output:
4

Explanation:
Replace the one ‘A’ in the middle with ‘B’ and form “AABBBBA”.
The substring “BBBB” has the longest repeating letters, which is 4.

2. 题意

在字符串中经过最多k个对字母的变换后，最长的重复子串长度

3. 思路

使用sliding window来处理。从l到r中间的子串中去掉最多出现的那个字母的出现次数，剩下的就是需要变换的字母数，如是大于k，就将l向右滑动，否则再更新max长度后，r再向右滑动。

4. 代码

class Solution(object):
    def characterReplacement(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: int
        """
        start=end=res=maxcur=0
        count=[0]*26
        while end<len(s):
            count[ord(s[end])-65]+=1
            maxcur=max(maxcur,max(count))
            while (end-start+1-maxcur)>k:
                count[ord(s[start])-65]-=1
                start+=1
            res=max(res,end-start+1)
            end+=1
        return res
        

5. Reference

http://www.programmersought.com/article/1764398713/