70. Climbing Stairs. Sol

也是斐波那契数列的变体。区别是初始项不同。如果直接用递归做会比较耗时

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

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

Example 1:

Input: n = 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps

Example 2:

Input: n = 3
Output: 3
Explanation: There are three ways to climb to the top.
1. 1 step + 1 step + 1 step
2. 1 step + 2 steps
3. 2 steps + 1 step

Constraints:

• 1 <= n <= 45

  class Solution:
      def climbStairs(self, n):
          if n<=2:
              return n
          else:
              prev1 = self.climbStairs(1)
              prev2 = self.climbStairs(2)
              for _ in range(n-2):
                  temp = prev2
                  prev2 = prev1 + prev2
                  prev1 = temp
              return prev2