3Sum

Find all unique triplets of values (a, b, c), taken from three different positions of the array, with a + b + c = 0.

Input:

• line 1: n
• line 2: n integers

Output:

• line 1: the number of unique triplets t
• next t lines: one triplet each, written in increasing order (a ≤ b ≤ c); list the triplets in increasing order (compare a, then b, then c)

Constraints: 3 ≤ n ≤ 3000, −10⁶ ≤ value ≤ 10⁶.

Think about it: sort, fix the first value, then find pairs with two pointers — O(n²). Skipping equal neighbours avoids duplicate triplets.