Swift Array extension Merge Sort

extension Array where Element: Comparable {
    
    func mergeSort() -> [Element] {
        if count <= 1 { return self }
        else {
            let left = Array(self[0 ..< count/2])
            let right = Array(self[count/2 ..< count])
            return merge(left.mergeSort(), right: right.mergeSort())
        }
    }
    
    func merge<T: Comparable>(var left:[T], var right:[T]) -> [T]{
        var list:[T] = []
        while left.count > 0 && right.count > 0 {
            if left[0] <= right[0] {
                list.append(left[0])
                left.removeFirst()
            }
            else {
                list.append(right[0])
                right.removeFirst()
            }
            
        }
        list.appendContentsOf(left)
        list.appendContentsOf(right)
        return list
    }
    
}