feat: Solution for August Challeng 25 - Minimum Cost For Tickets

Author: milanpanchal

Swift/August Challenge/25_MinimumCostForTickets.playground/Contents.swift

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+/*:
+#   Minimum Cost For Tickets
+
+ https://leetcode.com/explore/challenge/card/august-leetcoding-challenge/552/week-4-august-22nd-august-28th/3436/
+ 
+ ---
+
+### Problem Statement:
+ 
+ In a country popular for train travel, you have planned some train travelling one year in advance.  The days of the year that you will travel is given as an array `days`.  Each day is an integer from `1` to `365`.
+
+ Train tickets are sold in 3 different ways:
+
+ + a 1-day pass is sold for `costs[0]` dollars;
+ + a 7-day pass is sold for `costs[1]` dollars;
+ + 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: day 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.
+
+ ```
+
+ ### Notes:
+ + 1 <= days.length <= 365
+ + 1 <= days[i] <= 365
+ + days is in strictly increasing order.
+ + costs.length == 3
+ + 1 <= costs[i] <= 1000
+ 
+ */
+
+
+import UIKit
+
+class Solution {
+    
+//    66 / 66 test cases passed.
+//    Status: Accepted
+//    Runtime: 104 ms
+//    Memory Usage: 20.9 MB
+
+    func mincostTickets(_ days: [Int], _ costs: [Int]) -> Int {
+        
+        guard days.count > 0, costs.count > 0 else {
+            return 0
+        }
+        
+        var dp = Array(repeating: 0, count: days.last! + 1)
+        
+        for i in 1..<dp.count {
+            if days.contains(i) {
+
+                let option1 = dp[i - 1] + costs[0]          // daily
+                let option2 = dp[max(i - 7, 0)] + costs[1]  // weekly
+                let option3 = dp[max(i - 30, 0)] + costs[2] // monthly
+                
+                dp[i] = min(option1, option2, option3)
+
+            } else {
+                dp[i] = dp[i - 1]
+            }
+            
+        }
+        
+        return dp.last!
+    }
+
+    // Leetcode Solution
+    func mincostTickets2(_ days: [Int], _ costs: [Int]) -> Int {
+        var dp = Array(repeating:Int.max, count: days.count)
+        dp[0] = costs.min()!
+        let costs = zip([1, 7, 30], costs)
+        for (i, day) in days.enumerated().dropFirst() {
+            for (duration, cost) in costs {
+                var j = i
+                while j >= 0 && (day-days[j]) < duration {
+                    j -= 1
+                }
+                dp[i] = min(dp[i], (j >= 0 ? dp[j] : 0) + cost)
+            }
+        }
+        
+        return dp.last ?? 0
+    }
+}
+
+let sol = Solution()
+sol.mincostTickets([1,2,3,4,5,6,7,8,9,10,30,31], [2,7,15]) // 17
+sol.mincostTickets2([1,2,3,4,5,6,7,8,9,10,30,31], [2,7,15]) // 17

Swift/August Challenge/25_MinimumCostForTickets.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='ios'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>