70. Climbing Stairs.py

# 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?
# Note: Given n will be a positive integer.

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

# Example 2:
# Input: 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

class Solution(object):
    def climbStairs(self, n):
        """
        :type n: int
        :rtype: int
        """
        if n<= 1:
            return 1
        f = [None for _ in range(n+1)]
        f[0],f[1],f[2]=1,1,2
        for i in range(3,n+1):
            f[i]=f[i-1]+f[i-2]
        return f[n]

def main():
    s = Solution()
    print(s.climbStairs(2))
    print(s.climbStairs(3))

if __name__ == "__main__":
    main()