Maximum Subarray. Divide & Conquer. Θ(nlgn)

///Maximum Subarray. Divide & Conquer.  Θ(nlgn)
func maximumSubarray(_ array: [Int]) throws -> (Int, Int, Int) {
    return try _maximumSubarray(array, low: 0, high: array.count-1)
}

func _maximumSubarray(_ array: [Int], low:Int, high:Int) throws -> (Int, Int, Int) {
    //low>=0 NOT low>0
    guard low<=high, low>=0 else { throw TestError.argumentFault }
    if low == high { return (low, high, array[low]) }
    let mid = (low+high)/2
    let pre = try _maximumSubarray(array, low: low, high: mid)
    let post = try _maximumSubarray(array, low: mid+1, high: high)
    let cross = try crossSubarray(array, low: low, mid: mid, high: high)
    if pre.2>=post.2, pre.2>=cross.2 { return pre }
    if post.2>=pre.2, post.2>=cross.2 { return post }
    return cross
}

func crossSubarray(_ array:[Int], low:Int, mid:Int, high:Int) throws ->(Int, Int, Int) {
    //low>=0 NOT low>0
    guard low<=mid, mid<=high, low>=0 else { throw TestError.argumentFault }
    var maxLeft = Int.min, maxRight = Int.min, sum = 0, left = 0, right = 0
    for i in (low...mid).reversed() {
        sum += array[i]
        if sum > maxLeft {
            maxLeft = sum
            left = i
        }
    }
    sum = 0
    //Start from mid+1 NOT mid
    for i in mid+1...high {
        sum += array[i]
        if sum > maxRight {
            maxRight = sum
            right = i
        }
    }
    return (left, right, maxLeft+maxRight)
}

var s = try Int.randomArray(above: -10, under: 10)
try maximumSubarray(s)