From 321e83881fa840d209c0ef2aa2a1a6c6878b8683 Mon Sep 17 00:00:00 2001
From: Gutsonok <belars0809@gmail.com>
Date: Sat, 8 Aug 2020 23:38:05 +0500
Subject: [PATCH] Fix #140 Program crashes upon search for null in a
 SkipList<string>

---
 DataStructures/Lists/SkipList.cs             | 20 ++++++++++++++-
 UnitTest/DataStructuresTests/SkipListTest.cs | 27 ++++++++++++++++++--
 2 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/DataStructures/Lists/SkipList.cs b/DataStructures/Lists/SkipList.cs
index 4a990403..dbb57bc0 100644
--- a/DataStructures/Lists/SkipList.cs
+++ b/DataStructures/Lists/SkipList.cs
@@ -208,12 +208,31 @@ public bool Contains(T item)
         /// </summary>
         public bool Find(T item, out T result)
         {
+            result = default(T);
+
             var current = _firstNode;
 
+            // If find null element then check first element after first node
+            if (item == null)
+            {
+                current = current.Forwards[0];
+                return current != null && current.Value == null;
+            }
+
+            // Skip null element (in first postion) if contain
+            if (!IsEmpty && current.Forwards[0].Value == null)
+            {
+                current = current.Forwards[0];
+            }
+
             // Walk after all the nodes that have values less than the node we are looking for
             for (int i = _currentMaxLevel - 1; i >= 0; --i)
+            {
                 while (current.Forwards[i] != null && current.Forwards[i].Value.IsLessThan(item))
+                {
                     current = current.Forwards[i];
+                }
+            }   
 
             current = current.Forwards[0];
 
@@ -224,7 +243,6 @@ public bool Find(T item, out T result)
                 return true;
             }
 
-            result = default(T);
             return false;
         }
 
diff --git a/UnitTest/DataStructuresTests/SkipListTest.cs b/UnitTest/DataStructuresTests/SkipListTest.cs
index 244700fb..cbbe3e15 100644
--- a/UnitTest/DataStructuresTests/SkipListTest.cs
+++ b/UnitTest/DataStructuresTests/SkipListTest.cs
@@ -1,5 +1,4 @@
-using System.Runtime.InteropServices.ComTypes;
-using DataStructures.Lists;
+using DataStructures.Lists;
 using Xunit;
 
 namespace UnitTest.DataStructuresTests
@@ -16,6 +15,17 @@ public static void Initialization_ListIsEmpty()
             Assert.DoesNotContain(0, skipList);
         }
 
+        [Fact]
+        public static void InitializationWithReferencyTypeAsGeneric_ListIsEmpty()
+        {
+            var skipList = new SkipList<string>();
+
+            Assert.True(skipList.Count == 0);
+            Assert.True(skipList.IsEmpty);
+            Assert.DoesNotContain(null, skipList);
+            Assert.DoesNotContain(string.Empty, skipList);
+        }
+
         [Fact]
         public static void Add_NullElement_ListContainNullElement()
         {
@@ -27,6 +37,19 @@ public static void Add_NullElement_ListContainNullElement()
             Assert.Contains(null, skipList);
         }
 
+        [Fact]
+        public static void Add_NullElementAfterNonNull_ListContainNullElement()
+        {
+            var skipList = new SkipList<string>();
+
+            skipList.Add("1");
+            skipList.Add(null);
+
+            Assert.True(skipList.Count == 2);
+            Assert.Contains(null, skipList);
+            Assert.Contains("1", skipList);
+        }
+
         [Theory]
         [InlineData(10)]
         [InlineData(-10)]