Zero Triplets

Difficulty: 1600 3000 ms 128 MB

sorting two-pointers

Count the index triples i < j < k with a_i + a_j + a_k = 0.

Input: the first line has N; the second line has N integers.

Output: the number of zero-sum triples (by INDEX, so duplicates matter).

Constraints:

• 3 ≤ N ≤ 500
• |a_i| ≤ 30

Sample Tests

5
-1 0 1 2 -2

2

3
0 0 0

1

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