cs: add «857. Minimum Cost to Hire K Workers»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/minimum-cost-to-hire-k-workers.cs

@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+
+public class Solution {
+    private struct Worker(int quality, int wage) {
+        public int Quality => quality;
+        public int Wage => wage;
+        public double Ratio => (double)Wage / Quality;
+    }
+    private class WorkerComparator : IComparer<Worker> {
+        public int Compare(Worker lhs, Worker rhs) {
+            var l = lhs.Wage * rhs.Quality;
+            var r = rhs.Wage * lhs.Quality;
+            return l.CompareTo(r);
+        }
+    }
+
+    public double MincostToHireWorkers(int[] quality, int[] wage, int k) {
+        var workers = new List<Worker>(quality.Length);
+        for (var i = 0; i < quality.Length; ++i) {
+            workers.Add(new Worker(quality[i], wage[i]));
+        }
+        workers.Sort(new WorkerComparator());
+
+        double cost = double.MaxValue;
+        int currentQuality = 0;
+        var heap = new PriorityQueue<int, int>();
+
+        foreach (var worker in workers) {
+            currentQuality += worker.Quality;
+            heap.Enqueue(worker.Quality, -worker.Quality);
+
+            if (heap.Count > k) {
+                currentQuality -= heap.Dequeue();
+            }
+
+            if (heap.Count == k) {
+                cost = Math.Min(cost, currentQuality * worker.Ratio);
+            }
+        }
+
+        return cost;
+    }
+}