Three ways to implementation group() function.

SequenceType+group.swift

import Foundation

extension SequenceType where Generator.Element : Equatable {
    
    typealias E = Generator.Element
    
    // use reduce()
    func group() -> [[E]] {
        let initial: [[E]] = []
        let grouped = self.reduce(initial) { (var acc, x) -> [[E]] in
            if var lastGroup = acc.last where lastGroup.last == x {
                acc.removeLast()
                lastGroup.append(x)
                acc.append(lastGroup)
            } else {
                acc.append([x])
            }
            return acc
        }
        return grouped
    }
    
    // use forEach()
    func group2() -> [[E]] {
        var grouped: [[E]] = []
        self.forEach { (x) -> () in
            if var lastGroup = grouped.last where lastGroup.last == x {
                grouped.removeLast()
                lastGroup.append(x)
                grouped.append(lastGroup)
            } else {
                grouped.append([x])
            }
        }
        return grouped
    }
    
    // use for-in
    func group3() -> [[E]] {
        var grouped: [[E]] = []
        for x in self {
            if var lastGroup = grouped.last where lastGroup.last == x {
                grouped.removeLast()
                lastGroup.append(x)
                grouped.append(lastGroup)
            } else {
                grouped.append([x])
            }
        }
        return grouped
    }
}