Permutation in String: the idea to remember.

The core insight

Slide a fixed length window and compare its 26 character counts to the target.

Why it works

Only windows of exactly the target length can be permutations; equal frequency arrays are necessary and sufficient.

The mistake to avoid

Remove the outgoing character when the window exceeds the target length.

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.

boolean checkInclusion(String target, String text) {
    if (target.length() > text.length()) return false;
    int[] need = new int[26], window = new int[26];
    for (char c : target.toCharArray()) need[c - 'a']++;
    for (int r = 0; r < text.length(); r++) {
        window[text.charAt(r) - 'a']++;
        if (r >= target.length()) window[text.charAt(r - target.length()) - 'a']--;
        if (r + 1 >= target.length() && Arrays.equals(need, window)) return true;
    }
    return target.isEmpty();
}

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