public static boolean isPermutation(String s1, String s2) {
        if(s1.length() != s2.length()) {
            return false;
        }
        
        int[] counts1 = new int[256];
        int[] counts2 = new int[256];
        char[] chars1 = s1.toCharArray();
        char[] chars2 = s2.toCharArray();
        
        for(int i = 0; i < chars1.length; i++) {
            counts1[chars1[i]]++;
            counts2[chars2[i]]++;
        }
        
        return Arrays.equals(counts1, counts2);
    }