Explanation:
We have the option of two songs to begin with, so for the first branch the option is either OLD SONG or NEW SONG( in the out come, the top two start with O so that means the first song was OLD and since the bottom two start with N that means the first song was NEW)
THIS MEANS THE FIRST TOP BRANCH IS O AND THE BOTTOM IS N (song1)
Then, the second letter of the outcomes is the order in which you will write them on the branches for SONG number 2 the order from top to bottom is O,N,N,O (OLD SONG,NEW SONG, OLD SONG NEWSONG)