solution.java

class Solution {
    public boolean checkInclusion(String s1, String s2) {
        if (s1.length() > s2.length())
            return false;

        int[] s1Count = new int[26];
        int[] s2Count = new int[26];

        // Count the frequency of characters in s1 and the first window of s2
        for (int i = 0; i < s1.length(); i++) {
            s1Count[s1.charAt(i) - 'a']++;
            s2Count[s2.charAt(i) - 'a']++;
        }

        // Slide the window over s2
        for (int i = 0; i < s2.length() - s1.length(); i++) {
            if (matches(s1Count, s2Count))
                return true;
            // Update the window
            s2Count[s2.charAt(i) - 'a']--;
            s2Count[s2.charAt(i + s1.length()) - 'a']++;
        }

        // Check the last window
        return matches(s1Count, s2Count);
    }

    private boolean matches(int[] s1Count, int[] s2Count) {
        for (int i = 0; i < 26; i++) {
            if (s1Count[i] != s2Count[i])
                return false;
        }
        return true;
    }
}