mergesort.swift

func merge(a1: [Int], _ a2: [Int]) -> [Int] {
    // a1 and a2 are sorted already
    var ret: [Int] = []
    var i: Int = 0
    var j: Int = 0
    while i < a1.count || j < a2.count {
        if j == a2.count {
            if i == a1.count - 1 { 
                ret.append(a1[i]) 
            }
            else { 
                ret += a1[i...a1.count-1] 
            }
            break
        }
        if i == a1.count {
            if j == a2.count - 1 { 
                ret.append(a2[j]) 
            }
            else { 
                ret += a2[j...a2.count-1] 
            }
            break
        }
        if a1[i] < a2[j] { 
            ret.append(a1[i]); i += 1 
        }
        else { 
            ret.append(a2[j]); j += 1 
        }
    }
    return ret
}

func merge_sort(a: [Int]) -> [Int] {
    if a.count == 1 { 
        return a 
    }
    if a.count == 2 { 
        return merge([a[0]],[a[1]]) 
    }
    let i = a.count/2
    let a1 = merge_sort(Array(a[0...i]))
    let a2 = merge_sort(Array(a[i+1...a.count-1]))
    return merge(a1, a2)
}

func pp (s: String, _ a: [Int]) {
    print (s + " ")
    for n in a { print("\(n) ", terminator: " ") }
    print("")
}

let a = [32,7,100,29,55,3,19,82,23]
pp("before: ", a)
let m = merge_sort(a)
pp("merge : ", m)

/*
> swift test.swift
before:  
32  7  100  29  55  3  19  82  23  
merge :  
3  7  19  23  29  32  55  82  100  
>
*/