linq 递归

public class TreeNode
{
    public int Value { get; set; }
    public List<TreeNode> Children { get; set; } = new List<TreeNode>();
}

public static void Main()
{
    var root = new TreeNode { Value = 1 };
    var child1 = new TreeNode { Value = 2 };
    var child2 = new TreeNode { Value = 3 };
    var subChild = new TreeNode { Value = 4 };

    child1.Children.Add(subChild);
    root.Children.Add(child1);
    root.Children.Add(child2);

    // 使用LINQ递归遍历树
    var result = TraverseTree(root).ToList();

    foreach (var node in result)
    {
        Console.WriteLine(node.Value);
    }
}

public static IEnumerable<TreeNode> TraverseTree(TreeNode node)
{
    yield return node;
    foreach (var child in node.Children.SelectMany(child => TraverseTree(child)))
    {
        yield return child;
    }
}

public static IEnumerable<TreeNode> TraverseTreeIteratively(TreeNode root)
{
    var stack = new Stack<TreeNode>();
    stack.Push(root);

    while (stack.Count > 0)
    {
        var node = stack.Pop();
        yield return node;
        foreach (var child in node.Children.Reverse())
        {
            stack.Push(child);
        }
    }
}