LeetCode/cs/permutation-in-string.cs

public class Solution {
    private int[] GetFreqs(string s, int begin, int end) {
        var freqs = new int[26];

        for (int i = begin; i < end; ++i) {
            ++freqs[s[i] - 'a'];
        }

        return freqs;
    }
    private int[] GetFreqs(string s) => GetFreqs(s, 0, s.Length);

    public bool CheckInclusion(string s1, string s2) {
        if (s2.Length < s1.Length) {
            return false;
        }

        var expected = GetFreqs(s1);

        var running = GetFreqs(s2, 0, s1.Length);
        if (Enumerable.SequenceEqual(running, expected)) {
            return true;
        }

        for (int i = s1.Length; i < s2.Length; ++i) {
            --running[s2[i - s1.Length] - 'a'];
            ++running[s2[i] - 'a'];

            if (Enumerable.SequenceEqual(running, expected)) {
                return true;
            }
        }

        return false;
    }
}