0070.爬楼梯

https://leetcode.cn/problems/climbing-stairs/

方法一：动态规划

时间复杂度 $O(n)$，空间复杂度 $O(1)$。

func climbStairs(n int) int {
	if n < 3 {
		return n
	}
	first, second, three := 1, 2, 0
	for i := 3; i <= n; i++ {
		three = first + second
		first, second = second, three
	}
	return three
}