javadoc
added a-star example

Four people need to cross a rickety rope bridge to get back to their
camp at night. Unfortunately, they only have one torch and it only has
enough light left for seventeen minutes. The bridge is too dangerous to
cross without a torch, and it's only strong enough to support two people
at any given time. Each of the campers walks at a different speed. One
can cross the bridge in 1 minute, another in 2 minutes, the third in 5
minutes, and the fourth takes 10 minutes to cross. How do the campers
make it across in 17 minutes?