Update 404.左叶子之和，添加C#递归版

problems/0404.左叶子之和.md

@@ -651,6 +651,23 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+// 递归
+public int SumOfLeftLeaves(TreeNode root)
+{
+    if (root == null) return 0;
+
+    int leftValue = SumOfLeftLeaves(root.left);
+    if (root.left != null && root.left.left == null && root.left.right == null)
+    {
+        leftValue += root.left.val;
+    }
+    int rightValue = SumOfLeftLeaves(root.right);
+    return leftValue + rightValue;
+
+}
+```
 
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