983最低票價

• 983最低票價
• https://leetcode-cn.com/problems/minimum-cost-for-tickets/

class Solution(object):
    def mincostTickets(self, days, costs):
        if days is None or len(days) == 0 or costs is None or len(costs) == 0:
            return 0
        daysets = set(days)
        dp = [0 for i in range(days[len(days) - 1] + 1)]
        for i in range(1, len(dp)):
            if i not in daysets:
                dp[i] = dp[i - 1]
            else:
                # 如果要出行，肯定需要票，可能是1日票，7日票， 30日票
                n1 = dp[i - 1] + costs[0]
                n2 = costs[1] + dp[i - 7] if i >= 7 else costs[1]
                n3 = costs[2] + dp[i - 30] if i >= 30 else costs[2]
                dp[i] = min(n1, n2, n3)
        return dp[-1]