Longest Repeating Character Replacement: the idea to remember.

The core insight

A window needs length minus its largest character count replacements. Track a historical maximum frequency for a linear scan.

Why it works

The historical maximum can delay shrinking, but cannot create a larger best length without sufficient frequency having existed in an earlier window.

The mistake to avoid

With a historical maximum the current window need not be valid; use it for the maximum length, not to return a valid substring.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

int characterReplacement(String s, int k) {
    int[] count = new int[26];
    int left = 0, maxCount = 0, best = 0;
    for (int right = 0; right < s.length(); right++) {
        maxCount = Math.max(maxCount, ++count[s.charAt(right) - 'A']);
        while (right - left + 1 - maxCount > k) {
            count[s.charAt(left++) - 'A']--;
        }
        best = Math.max(best, right - left + 1);
    }
    return best;
}

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