114. Flatten Binary Tree to Linked List

114. Flatten Binary Tree to Linked List/Program.cs

@@ -0,0 +1,67 @@
+var sol = new Solution();
+
+var cases = new (int?[] input, int?[] expected)[]
+{
+	(new int?[] { 1, 2, 5, 3, 4, null, 6 }, new int?[] { 1, null, 2, null, 3, null, 4, null, 5, null, 6 }),
+	// (Array.Empty<int?>(), Array.Empty<int?>()),
+	// (new int?[] { 0 }, new int?[] { 0 })
+};
+
+foreach (var (input, expected) in cases)
+{
+	var root = BuildTreeFromLevelOrder(input);
+	if (root != null)
+		sol.Flatten(root);
+
+	var actual = FlattenToOutputList(root);
+	Console.WriteLine($"[{string.Join(", ", input.Select(FormatNullable))}] -> [{string.Join(", ", actual.Select(FormatNullable))}] (expected: [{string.Join(", ", expected.Select(FormatNullable))}])");
+}
+
+static TreeNode? BuildTreeFromLevelOrder(int?[] data)
+{
+	if (data.Length == 0 || data[0] == null)
+		return null;
+
+	var root = new TreeNode(data[0].Value);
+	var queue = new Queue<TreeNode>();
+	queue.Enqueue(root);
+
+	var i = 1;
+	while (queue.Count > 0 && i < data.Length)
+	{
+		var node = queue.Dequeue();
+
+		if (i < data.Length && data[i] != null)
+		{
+			node.left = new TreeNode(data[i]!.Value);
+			queue.Enqueue(node.left);
+		}
+		i++;
+
+		if (i < data.Length && data[i] != null)
+		{
+			node.right = new TreeNode(data[i]!.Value);
+			queue.Enqueue(node.right);
+		}
+		i++;
+	}
+
+	return root;
+}
+
+static List<int?> FlattenToOutputList(TreeNode? root)
+{
+	var result = new List<int?>();
+	var node = root;
+	while (node != null)
+	{
+		result.Add(node.val);
+		if (node.right != null)
+			result.Add(null);
+		node = node.right;
+	}
+
+	return result;
+}
+
+static string FormatNullable(int? value) => value?.ToString() ?? "null";