boolean isPermutation3(String s1, String s2) {
		if (s1.length() != s2.length()) {
			return false;
		}

		int[] letters = new int[256];
		for (int i = 0; i < s1.length(); i++) {
			letters[s1.charAt(i)]++;
		}
		for (int i = 0; i < s2.length(); i++) {
			letters[s2.charAt(i)]--;
			if (letters[s2.charAt(i)] < 0) {
				return false;
			}
		}
		return true;
	}