新增 哈希表部分的 C#版

problems/0454.四数相加II.md

@@ -318,5 +318,32 @@ impl Solution {
 }
 ```
 
+C#：
+```csharp
+public int FourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
+       Dictionary<int, int> dic = new Dictionary<int, int>();
+        foreach(var i in nums1){
+            foreach(var j in nums2){
+                int sum = i + j;
+                if(dic.ContainsKey(sum)){
+                    dic[sum]++;
+                }else{
+                    dic.Add(sum, 1);
+                }
+                
+            }
+        }
+        int res = 0;
+         foreach(var a in nums3){
+            foreach(var b in nums4){
+                int sum = a+b;
+                if(dic.TryGetValue(-sum, out var result)){
+                    res += result;
+                }
+            }
+        }
+        return res;
+    }
+```
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