Add csharp solution for Top K Frequent Elements

Author: SPponmag

Diff for: csharp/347-Top-K-Frequent-Elements.cs

@@ -0,0 +1,29 @@
+public class Solution {
+    public int[] TopKFrequent(int[] nums, int k) {
+        int[] arr = new int[k];
+        var dict = new Dictionary<int, int>();
+        for (int i = 0; i < nums.Length; i++)
+        {
+            if (dict.ContainsKey(nums[i]))
+            {
+                dict[nums[i]]++;
+            }
+            else
+            {
+                dict.Add(nums[i], 1);    
+            }
+        }
+        
+        var pq = new PriorityQueue<int, int>();
+        foreach (var key in dict.Keys)
+        {
+            pq.Enqueue(key, dict[key]);
+            if (pq.Count > k) pq.Dequeue();
+        }
+        int i2 = k;
+        while (pq.Count > 0) {
+            arr[--i2] = pq.Dequeue();
+        }
+        return arr;
+    }
+}