def climb_stairs(n):
    if n <= 2:
        return n
    return climb_stairs(n-1) + climb_stairs(n-2)

def climb_stairs(n):
    if n < 1:
        raise ValueError("n必须为正整数")
    dp = [] * (n + 1)
    dp[1], dp[2] = 1, 2
    for i in range(3, n + 1):
        dp[i] = dp[i-1] + dp[i-2]
    return dp[n]

def climb_stairs(n):
    a, b = 1, 2
    for _ in range(n):
        a, b = b, a + b
    return a

def climb_stairs(n):
    return 2 ** (n-1)