其它
Two men and two boys are planning to cross a river using a small boat that can only hold either one man or two boys. What is the least possible number of times the boat needs to cross the river in order to bring all four of them to the other side? 
Overseas Competition->Knowledge Point->Combinatorics->Strategies and Operations->Operational Problem
Two boys cross the river and one of them goes back.  A man crosses the river and the other boy goes back.  Two boys cross the river and one of them goes back.  The other man crosses the river and the other boy goes back.  Two boys cross the river. 