759. Employee Free Time

 func employeeFreeTime(_ schedule: [[Interval]]) -> [Interval] {
        var res = [Interval]()
        var map = [(Int, Bool)]()  // timestamp, isStart
        for sche in schedule{
            for inter in sche {
                map.append((inter.start, true))
                map.append((inter.end, false))
            }
        }
        map.sort{
            if $0.0 == $1.0 {
                return $0.1 ? false : true
            }
            return $0.0 < $1.0
        }

        var i = 0
        var preEnd = -1
        var startCount = 0
        for i in  0 ..< map.count{
            let cur = map[i]
            if cur.1 {
                if preEnd != -1 && preEnd != cur.0 && startCount == 0{
                    res.append(Interval(preEnd, cur.0))
                }
                startCount += 1
            }else{
                preEnd = max(preEnd, cur.0)
                startCount -= 1
            }
        }
        return res
    }