3SUM problem raw algorithm with O(n²) at worst

3SUM.swift

extension Collection where Element: Comparable & Numeric {
    typealias Triplet = (Element, Element, Element)
    
    func threeSum() -> [Triplet] {
        var result = [Triplet]()
        let sortedSelf = sorted()
        for (offset, a) in sortedSelf.dropLast().enumerated() {
            var slice = sortedSelf.dropFirst(offset + 1)
            while slice.count > 1, let b = slice.first, let c = slice.last {
                if a + b + c == 0 {
                    result.append((a, b, c))
                    slice = slice.dropFirst().dropLast()
                } else if a + b + c > 0 {
                    slice = slice.dropLast()
                } else {
                    slice = slice.dropFirst()
                }
            }
        }
        return result
    }
}