//Hash
func disappearedNumbers(nums: [Int]) -> [Int] {
    if nums.isEmpty {
        return []
    }
    var set = Set(nums)
    var missingNumms = [Int]()
    for i in 1...nums.count {
        if !set.contains(i) {
            missingNumms.append(i)
        }
    }
    return missingNumms
}

let nums = disappearedNumbers(nums: [4,3,2,7,8,2,3,1])
print(nums)