Write a method that takes two lists L1 and L2 of equal length and returns a new list where L1 is reordered to match the order of L2.

The approach is to map each element of L2 to its index and then sort L1 using that mapping. In C#:

List<int> Reorder(List<int> l1, List<int> l2)
{
    var indexMap = l2.Select((v, i) => new { v, i })
                     .ToDictionary(x => x.v, x => x.i);
    return l1.OrderBy(x => indexMap[x]).ToList();
}

This solution has O(n log n) time complexity because of the ordering operation and O(n) space for the dictionary. If the input lists are large and performance critical, you can avoid sorting by creating an output array and placing each element of L1 at the position indicated by the mapping, which runs in O(n) time.