2361. Minimum Costs Using the Train Line

A train line going through a city has two routes, the regular route and the express route. Both routes go through the same n + 1 stops labeled from 0 to n. Initially, you start on the regular route at stop 0.

You are given two 1-indexed integer arrays regular and express, both of length n. regular[i] describes the cost it takes to go from stop i - 1 to stop i using the regular route, and express[i] describes the cost it takes to go from stop i - 1 to stop i using the express route.

You are also given an integer expressCost which represents the cost to transfer from the regular route to the express route.

Note that:

• There is no cost to transfer from the express route back to the regular route.
• You pay expressCost every time you transfer from the regular route to the express route.
• There is no extra cost to stay on the express route.

Return a 1-indexed array costs of length n, where costs[i] is the minimum cost to reach stop i from stop 0.

Note that a stop can be counted as reached from either route.

class Solution {
public:
    vector<long long> minimumCosts(vector<int>& regular, vector<int>& express, int expressCost) {
        vector<long long> res;
        long long rdp = 0, edp = expressCost;
        for(int i = 0; i < regular.size(); i++) {
            long long rdpp = min(rdp, edp) + regular[i];
            long long edpp = min(rdp + expressCost, edp) + express[i];
            
            rdp = rdpp;
            edp = edpp;
            
            res.push_back(min(rdp, edp));
        }
        
        return res;
    }
};