424. Longest Repeating Character Replacement

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424. Longest Repeating Character Replacement

题目描述

Given a string that consists of only uppercase English letters, you can replace any letter in the string with another letter at most k times. Find the length of a longest substring 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:

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Input:
s = "ABAB", k = 2
Output:
4
Explanation:
Replace the two 'A's with two 'B's or vice versa.

Example 2:

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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.

题目大意

替换字符串 s 中 k 个字符，使字符串拥有最长重复字符。

解题思路

滑动窗口
维护有效区间 l...r，使得区间内除重复字符以外的字符不大于k。

代码

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class Solution {
   func characterReplacement(_ s: String, _ k: Int) -> Int {
        var dict = Array(repeating: 0, count: 26)
        var ans = 0
        var l = 0
        var chars = Array(s.unicodeScalars)
        var maxCount = 0
        for (r, char) in chars.enumerated() {
            let ascii = Int(char.value) - 65
            dict[ascii] += 1
            maxCount = max(maxCount, dict[ascii])
            while r - l + 1 - dict[Int(chars[l].value) - 65] > k {
                dict[Int(chars[l].value) - 65] -= 1
                l += 1
            }
        }
        return min(chars.count, maxCount + k)
    }
}
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