Merge two sorted array into one in the same order

Merge Sorted.swift

func mergeSorted<T:Comparable>(_ x: [T], _ y: [T], inOrder cmp: (T, T) -> Bool = (<) ) -> [T] {
    var z = [T]()
    var i = 0, j = 0
    while i < x.count || j < y.count {
        if j == y.count || cmp(x[i], y[j]) {
            z.append(x[i])
            i += 1
        } else {
            z.append(y[j])
            j += 1
        }
    }
    return z
}

let a, b: [Int]
a = [3,5,7,20,21]
// a = [5,3]
b = [1,7,9,11]
// b = [4]

print(mergeSorted(a,b))
// print(mergeSorted(a,b,inOrder: >))