Problem3:

An ordinary adjacency list has a link list for each vertex in the graph. A
node in the link list has a pointer to the next node. In the question you
are ask to make two occurences of each edge point to each other. You can add
one additional pointer to the node and point it to the node which represents
the same edge.

For example, 1--3 is an edge between vertex 1 and vertex 3. Add the pointers
so that they point to each other.

1 -> ...-> 3 -> ... ->/
          / \
2 -> ... |   |
          \ /
3 -> ...-> 1 -> ... ->/

4 -> ...
.
.
.