static boolean isPermutationCount(String s1, String s2) {
        if (s1.length() != s2.length()) {
            return false;
        }
        int[] charCount = new int[256];

        for (int i = 0; i<s1.length(); i++) {
            charCount[s1.charAt(i)]++;
        }

        for (int i = 0; i<s2.length(); i++) {
            --charCount[s2.charAt(i)];
        }
        int sum = 0;
        for (int c: charCount) {
            sum += c;
        }
        return sum==0;
    }