function encode(lines)
        {
            var text = "";
            for (var i = 0; i < lines.length; ++i) {
                var line = lines[i];
                var character = lineToChar.get(line);
                if (!character) {
                    character = String.fromCharCode(charCode++);
                    lineToChar.set(line, character);
                }
                text += character;
            }
            return text;
        }
@param {!Array.<string>} lines
@return {string}