Creating array of lists and combining

Question

 I want to do the following:

 

1)Create an array of 100 lists

 

So, I have:

 

List<int>[] a = new List<int>[100];

2)Next I do,

 

for(j=0; j<100;j++)
{

a[j] = new List<int>();


}

3) Next I add elements to each;

 

a[j].Add(3);

//and so on

4) Now I want to combine all the lists into a single list

 

I do not know how to do that

 

Please note that I cannot create a single list due to some algorithmic constarints.

 

Can anyone advise?