cs: add «404. Sum of Left Leaves»

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/sum-of-left-leaves.cs

@@ -0,0 +1,34 @@
+#if _MF_TEST
+// Definition for a binary tree node.
+public class TreeNode {
+    public int val;
+    public TreeNode left;
+    public TreeNode right;
+    public TreeNode(int val = 0, TreeNode left = null, TreeNode right = null) {
+        this.val = val;
+        this.left = left;
+        this.right = right;
+    }
+}
+#endif
+
+public class Solution {
+    private static bool IsLeaf(TreeNode node)
+        => node != null && node.left == null && node.right == null;
+
+    private int Sum(TreeNode node, bool isLeft) {
+        if (node == null) {
+            return 0;
+        }
+
+        if (isLeft && IsLeaf(node)) {
+            return node.val;
+        }
+
+        return Sum(node.left, true) + Sum(node.right, false);
+
+    }
+
+    public int SumOfLeftLeaves(TreeNode root)
+        => Sum(root, false);
+}