leetcode-164周赛-1269-停在原地的方案数

题目描述：

自己的提交：

class Solution:
    def numWays(self, steps: int, arrLen: int) -> int:
        l = min(steps,arrLen)
        dp = [0] * l
        dp[0] = 1
        MOD = 10 ** 9 + 7
        for step in range(steps):
            dp_ = dp[:]
            for i in range(len(dp)):
                if i == 0:
                    dp_[i] = (dp[i] + dp[i+1]) % MOD
                elif i == len(dp) - 1:
                    dp_[i] = (dp[i] + dp[i-1]) % MOD
                else:
                    dp_[i] = (dp[i-1] + dp[i] + dp[i+1]) % MOD
            dp = dp_
        return dp[0]

另：

class Solution:
    def numWays(self, steps: int, arrLen: int) -> int:
        l = min(steps,arrLen)
        dp = [0] * l
        dp[0] = 1
        MOD = 10 ** 9 + 7
        for step in range(steps):
            dp_ = [0] * l
            for i in range(len(dp)):
                for j in [-1,0,1]:
                    if 0 <= i+j < l:
                        dp_[i] += dp[i+j]
                dp_[i] %= MOD
            dp = dp_
 
        return dp[0]