FIBO 时间复杂度 O(N)
每次只能爬一步或两步, 则第n级台阶要么从第 n - 1级台阶跨一步,要么从第n - 2级台阶跨两步
即 f(n) = f(n - 1) + f(n - 2)
class Solution:
def climbStairs(self, n: int) -> int:
if n <= 2:
return n
a, b = 1, 2
for _ in range(3, n + 1):
a, b = b, a + b
return b