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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0901_1000.s0983_minimum_cost_for_tickets;

// #Medium #Array #Dynamic_Programming #2022_03_31_Time_1_ms_(93.95%)_Space_42.5_MB_(28.58%)

/**
 * 983 - Minimum Cost For Tickets\.
 *
 * Medium
 *
 * You have planned some train traveling one year in advance. The days of the year in which you will travel are given as an integer array `days`. Each day is an integer from `1` to `365`.
 *
 * Train tickets are sold in **three different ways**:
 *
 * *   a **1-day** pass is sold for `costs[0]` dollars,
 * *   a **7-day** pass is sold for `costs[1]` dollars, and
 * *   a **30-day** pass is sold for `costs[2]` dollars.
 *
 * The passes allow that many days of consecutive travel.
 *
 * *   For example, if we get a **7-day** pass on day `2`, then we can travel for `7` days: `2`, `3`, `4`, `5`, `6`, `7`, and `8`.
 *
 * Return _the minimum number of dollars you need to travel every day in the given list of days_.
 *
 * **Example 1:**
 *
 * **Input:** days = [1,4,6,7,8,20], costs = [2,7,15]
 *
 * **Output:** 11
 *
 * **Explanation:** For example, here is one way to buy passes that lets you travel your travel plan:
 *
 * On day 1, you bought a 1-day pass for costs[0] = $2, which covered day 1.
 *
 * On day 3, you bought a 7-day pass for costs[1] = $7, which covered days 3, 4, ..., 9.
 *
 * On day 20, you bought a 1-day pass for costs[0] = $2, which covered day 20.
 *
 * In total, you spent $11 and covered all the days of your travel.
 *
 * **Example 2:**
 *
 * **Input:** days = [1,2,3,4,5,6,7,8,9,10,30,31], costs = [2,7,15]
 *
 * **Output:** 17
 *
 * **Explanation:** For example, here is one way to buy passes that lets you travel your travel plan:
 *
 * On day 1, you bought a 30-day pass for costs[2] = $15 which covered days 1, 2, ..., 30.
 *
 * On day 31, you bought a 1-day pass for costs[0] = $2 which covered day 31.
 *
 * In total, you spent $17 and covered all the days of your travel.
 *
 * **Constraints:**
 *
 * *   `1 <= days.length <= 365`
 * *   `1 <= days[i] <= 365`
 * *   `days` is in strictly increasing order.
 * *   `costs.length == 3`
 * *   `1 <= costs[i] <= 1000`
**/
public class Solution {
    public int mincostTickets(int[] days, int[] costs) {
        int[] memo = new int[days.length + 1];
        memo[memo.length - 1] = 0;
        for (int i = days.length - 1; i >= 0; i--) {
            memo[i] =
                    Math.min(
                            Math.min(
                                    costs[0] + memo[getNext(days, i, days[i])],
                                    costs[1] + memo[getNext(days, i, days[i] + 6)]),
                            costs[2] + memo[getNext(days, i, days[i] + 29)]);
        }
        return memo[0];
    }

    public static int getNext(int[] days, int index, int goodUntil) {
        while (index < days.length && days[index] <= goodUntil) {
            index++;
        }
        return index;
    }
}