feat(csharp): add top_k (#554)

codes/csharp/chapter_heap/top_k.cs

@@ -0,0 +1,36 @@
+/**
+* File: top_k.cs
+* Created Time: 2023-06-14
+* Author: hpstory (hpstory1024@163.com)
+*/
+
+namespace hello_algo.chapter_heap; 
+
+public class top_k {
+    /* 基于堆查找数组中最大的 k 个元素 */
+    public static PriorityQueue<int, int> topKHeap(int[] nums, int k) {
+        PriorityQueue<int, int> heap = new PriorityQueue<int, int>();
+        // 将数组的前 k 个元素入堆
+        for (int i = 0; i < k; i++) {
+            heap.Enqueue(nums[i], nums[i]);
+        }
+        // 从第 k+1 个元素开始，保持堆的长度为 k
+        for (int i = k; i < nums.Length; i++) {
+            // 若当前元素大于堆顶元素，则将堆顶元素出堆、当前元素入堆
+            if (nums[i] > heap.Peek()) {
+                heap.Dequeue();
+                heap.Enqueue(nums[i], nums[i]);
+            }
+        }
+        return heap;
+    }
+
+    [Test]
+    public void Test() {
+        int[] nums = { 1, 7, 6, 3, 2 };
+        int k = 3;
+        PriorityQueue<int, int> res = topKHeap(nums, k);
+        Console.WriteLine("最大的 " + k + " 个元素为");
+        PrintUtil.PrintHeap(res);
+    }
+}