Climbing Stairs

You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

思路。

f(n)表示爬到n层楼梯的方法。f(n) = f(n-1)+f(n-2)。因此，可以用递归和迭代处理，递归中很多的重复计算，效率太慢。使用迭代，时间复杂度O(n),空间复杂度O(1）。

class Solution{
public:
	static int climb(int n)
	{
		int pre = 0;
		int cur = 1;
		for(int i = 1;i <=n;++i)
		{
			int tmp = cur;
			cur+=pre;
			pre=tmp;
		}
		return cur;
	}
};