For each virus node, count how many normal it connects. By doing dfs, if a normal connects to another virus node, then branch from this normal node won’t count.
In below case, there are 2 virus node [0, 5]. node 1 can connect with [1, 2, 8, 9]. 5 can connect with [6, 7]. [3, 4] connects to 2 virus node, so they won’t be counted.
