For a deeper audit, the committee wants to know how many GROUPS of three marks sum to exactly zero after marks were normalized around the class average (so some values are negative). Read N integers. Count the number of index triplets (i<j<k) such that a[i]+a[j]+a[k] = 0. Aim for O(N^2) using sorting + two pointers, not O(N^3).
Input
Line1: N. Line2: N integers.
Output
Count of triplets summing to zero.
Constraints
1 ≤ N ≤ 3000, |a_i| ≤ 10^4
Sample Input
6 -1 0 1 2 -1 -4
Sample Output
2
main.c
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