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public class Solution {
    private boolean check(int[] arr) {
        for (int num : arr) {
            if (num != 0) return false;
        }
        return true;
    }
    public boolean checkInclusion(String s1, String s2) {
        if (s1 == null || s2 == null) return false;
        int len1 = s1.length();
        int len2 = s2.length();
        if (len1 > len2) return false;
        int[] vector = new int[26];
        int count = len1;
        for(int i = 0; i < len1; i++) {
            vector[s1.charAt(i) - 'a']++;
            vector[s2.charAt(i) - 'a']--;
        }
        if (check(vector)) return true;
        for (int i = 0; i + len1 < len2; i++) {
            vector[s2.charAt(i) - 'a']++;
            vector[s2.charAt(i + len1) - 'a']--;
             if (check(vector)) return true;
        }
        return false;
    }
}