is it possible to program an arduino with usb/ttl adapter
The Arduino onboard USB to serial is connected to D0 (Rx) and D1 (Tx).

And without a USB connected to the Arduino you can use a USB/TTL adapter connected to D0 and D1 like this;

Arduino  - USB/TTL
Tx       - Rx
Rx       - Tx


And to get the Arduino inte "programming" mode:


Automatic way, if the USB/TTL have CTS, connect it to Arduinos reset pin.
Manual way, press the reset button when the Arduino IDE starts to upload the code.