function commonPrefixLength(a, b) {
        var i, len = Math.min(a.length, b.length);
        for (i = 0; i < len; i++) {
            if (a.charCodeAt(i) !== b.charCodeAt(i)) {
                return i;
            }
        }
        return len;
    }
@returns the length of the common prefix of the two strings.