There are N steps.ur at the No.Y step.How many ways would it be if u walk K steps?one step once.

问题描述:

There are N steps.ur at the No.Y step.How many ways would it be if u walk K steps?one step once.
There are N steps.ur at the No.Y step.How many ways would it be if u walk K steps?one step one time.Who's smart enough to help me solve this problem?Help please.

It would be two ways---upstairs or downstairs.
两种情况:上楼、下楼.