problems(cs): add „814. Binary Tree Pruning“

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

problems/binary-tree-pruning.cs

@@ -0,0 +1,36 @@
+/**
+ * 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;
+ *     }
+ * }
+ */
+public class Solution {
+    private static bool PruneTreeRec(TreeNode? node) {
+        if (node == null) {
+            return false;
+        }
+
+        if (!PruneTreeRec(node.left)) {
+            node.left = null;
+        }
+        if (!PruneTreeRec(node.right)) {
+            node.right = null;
+        }
+
+        return node.val == 1 || node.left != null || node.right != null;
+    }
+
+    public TreeNode? PruneTree(TreeNode? root) {
+        if (!PruneTreeRec(root)) {
+            return null;
+        }
+        return root;
+    }
+}