Given 4 lists of numbers (including -ve no.s) of length n, find all combinations of numbers (a, b, c, d) from respective lists which sum to zero.

We need to find 4 numbers, one from each list which sum up to zero.

Solving for 2 lists:
We can solve for 2 lists in O(n) as follows,

• Put the first list in a hashmap.
• Iterate the second list
• for each element s(i) in second list, lookup "0-s(i)" in the map
• If found we got one pair

Solving for 4 lists:

• Combine first and second list as follows,
• For each combination of f(i) and s(j), create an element f(i)+s(j)
• This gives us n^2 elements
• Combine 3rd and 4th lists in a similar manner and get another n^2 list.
• Now we can solve for these two lists of size n^2 in O(n^2).