I am unable to solve
under given constraints. Can anyone can provide hints/ideas/or solutions on how to solve this?
In this problem you are given a sequence of N positive integers S1,S, … S[N].
In addition you are given an integer T, and your aim is to find the number of quadruples (i, j, k, l),
such that 1 <= i < j < k < l <= N, and S[i] + S[j] + S[k] + S[l] = T.
That is, the number of ways of picking four numbers from the sequence summing up to T.
1 <= N <= 5000
1 <= T <= 1000000
1 <= S[i] <= 1000000000 , for all i.