need help with some explanation

[zivoni]

Before your for loop. adjList is initialized as [[], [], [], []]. In every pass of a loop one line is read from file, converted to a pair of ints and "processed".

First line is 0 1, that means there is an edge connecting vertex 0 with vertex 1, so 1 is appended to adj[0] list (to note that vertex 0 is connected to vertex 1), and 0 is appended to adj[1] list (again, to note that 1 is connected to 0). So after first run of loop block adjList is [[1], [0], [], []].

In second run edge 2 1 is added, so adjList becomes [[1], [0, 2], [1], []]. In remaining two runs edges 0 2 and 1 3 are added, so when for loop ends, adjList is [[1, 2], [0, 2, 3], [1, 0], [1]]

And , "need help with some explanation" is a terrible thread title, title actually saying something would be much better - for example "making graph from file" or "how edges convert to a list".