983. Minimum Cost For Tickets 买票的最小花费

class Solution {
public:
    int mincostTickets(vector<int>& days, vector<int>& costs) {
         dp.insert({-30, 0});
        for(int d : days)
            dp[d]=min({costs[0]+cost(d-1), costs[1]+cost(d-7), costs[2]+cost(d-30)});
        return dp.rbegin()->second;
    }
    
    int cost(int d) {
        return prev(dp.upper_bound(d))->second;
    }
    map<int, int> dp;
};