solution.cpp

class Solution
{
public:
    bool checkInclusion(string s1, string s2)
    {
        if (s1.length() > s2.length())
            return false;

        vector<int> s1Count(26, 0), s2Count(26, 0);

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

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

        // Check the last window
        return s1Count == s2Count;
    }
};