using System;
using System.Collections.Generic;
using System.Linq;
static class Program
{
public static void Main(string[] args)
{
var list1 = new int?[] {1, 2, 3, 4, 5};
var list2 = new int?[] {3, 4, 5, 6, 7};
var list3 = new int?[] {6, 9, 9};
var lockstep = LockStepSequences(new[] {list1, list2, list3});
foreach (var step in lockstep)
Console.WriteLine(string.Join("\t", step.Select(i => i.HasValue? i.Value.ToString() : "null").ToArray()));
}
public static IEnumerable<IEnumerable<T>> LockStepSequences<T>(this IEnumerable<IEnumerable<T>> sequences)
{
var iters = sequences
.Select((s, index) => new {index, enumerator = s.GetEnumerator()})
.Where(it => it.enumerator.MoveNext())
.ToArray();
try
{
for(var active = iters.ToList(); active.Any();)
{
T min = active.Min(it => it.enumerator.Current);
var corresponding = active.Where(it => Equals(it.enumerator.Current, min));
yield return iters.Select((el, i) => corresponding.Any(it => it.index == i)
? el.enumerator.Current
: default(T));
active = active.Except(corresponding.Where(it => !it.enumerator.MoveNext())).ToList();
}
}
finally
{
foreach (var iter in iters) iter.enumerator.Dispose();
}
}
}
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