114. Flatten Binary Tree to Linked List

114. Flatten Binary Tree to Linked List/114. Flatten Binary Tree to Linked List.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <RootNamespace>_114._Flatten_Binary_Tree_to_Linked_List</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

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

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+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";

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

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+
+public class Solution
+{
+    public void Flatten(TreeNode? root)
+    {
+        if (root == null)
+            return;
+        var head = root;
+        var stack = new Stack<TreeNode>();
+        if (root.right != null)
+            stack.Push(root.right);
+        if (root.left != null)
+            stack.Push(root.left);
+        while (stack.Count > 0)
+        {
+            var node = stack.Pop();
+            head.left = null;
+            head.right = node;
+            head = node;
+            if (node.right != null)
+                stack.Push(node.right);
+            if (node.left != null)
+                stack.Push(node.left);
+
+        }
+    }
+}

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

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+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;
+    }
+}