Modify the problem of preorder_traversal_iii

codes/csharp/chapter_backtracking/preorder_traversal_iii_compact.cs

@@ -21,6 +21,7 @@ public class preorder_traversal_iii_compact {
         if (root.val == 7) {
             // 记录解
             res.Add(new List<TreeNode>(path));
+            return;
         }
         preOrder(root.left);
         preOrder(root.right);