424. Longest Repeating Character Replacement

Sliding Window

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class Solution {
    public int characterReplacement(String s, int k) {
        // window: [i, j]
        int[] windowMap = new int[26];

        int maxLength = 0;

        int i = 0;
        for (int j = 0; j < s.length(); j++) {
            windowMap[s.charAt(j) - 'A']++;
            // 移除元素直到 window 满足条件
            while (windowIllegal(windowMap, k)) {
                windowMap[s.charAt(i) - 'A']--;
                i++;
            }

            maxLength = Math.max(maxLength, j - i + 1);
        }

        return maxLength;
    }

    private boolean windowIllegal(int[] windowMap, int k) {
        int charCount = 0, mostCharCount = 0;
        for (int count : windowMap) {
            charCount += count;
            mostCharCount = Math.max(mostCharCount, count);
        }

        return charCount - mostCharCount > k;
    }
}

Sliding Window

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class Solution {
    public int characterReplacement(String s, int k) {
        // window: [i, j]
        int[] windowMap = new int[26];

        int mostCharCount = 0, maxLength = 0;

        int i = 0;
        for (int j = 0; j < s.length(); j++) {
            windowMap[s.charAt(j) - 'A']++;
            mostCharCount = Math.max(mostCharCount, windowMap[s.charAt(j) - 'A']);

            if (j - i + 1 > mostCharCount + k) {
                windowMap[s.charAt(i) - 'A']--;
                i++;
            }

            maxLength = Math.max(maxLength, j - i + 1);
        }

        return maxLength;
    }
}

References

424. Longest Repeating Character Replacement