Check out example codes for "C# walk down a tree and back". It will help you in understanding the concepts better.

Code Example 1

public static IEnumerable<T> Traverse<T>(T item, Func<T, IEnumerable<T>> childSelector)
{
    var stack = new Stack<T>();
    stack.Push(item);
    while (stack.Any())
    {
        var next = stack.Pop();
        yield return next;
        foreach (var child in childSelector(next))
            stack.Push(child);
    }
}

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