1011. Capacity To Ship Packages Within D Days

func shipWithinDays(_ weights: [Int], _ D: Int) -> Int {
        var start = Int.max
        var end = 0
        for w in weights{
            start = min(start, w)
            end += w
        }

        //guess the w
        while start + 1 < end{
            let mid = (start + end) / 2
            if !canShip(weights, D, mid) {  //如果不能ship 说明给的weight太小
                start = mid
            }else{
                end = mid
            }
        }
        if canShip(weights, D, start) { return start}
        return end
    }


    func canShip(_ arr: [Int], _ D: Int, _ weight: Int) -> Bool{
        var curw = 0
        var count = 1
        for w in arr{
            if curw + w > weight{
                count += 1
                curw = w
                if count > D || curw > weight{
                    return false
                }
            }else{
                curw += w
            }
        }
        return true
    }