Fix aalhour#140 Program crashes upon search for null in a SkipList<st…

…ring>
Gutsonok · Aug 8, 2020 · 321e838 · 321e838
1 parent 377a8d1
commit 321e838
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Showing 2 changed files with 44 additions and 3 deletions.
diff --git 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
@@ -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)]