A method of decoding a bitstream by an electronic device is provided. A block unit is determined from an image frame according to the bitstream. A plurality of candidate modes included in a mode list are separated in a first mode group and a second mode group. The electronics device determines whether a specific one of the candidate modes in the first mode group is replaced by one of the candidate modes in the second mode group. When the specific candidate mode in the first mode group is replaced, a prediction mode is determined from the candidate modes in the second mode group. When the specific candidate mode in the first mode group remains unchanged, the specific candidate mode in the first mode group is determined as the prediction mode. Then, the block unit of the image frame is reconstructed based on the prediction mode.