Leetcode #1011: Capacity To Ship Packages Within D Days

func shipWithinDays(_ weights: [Int], _ D: Int) -> Int {
    var start = weights.max()!
    var end = weights.reduce(0,+)
    while start < end {
        let mid = start + (end - start) / 2
        let days = shipDays(weights, mid)
        if days == D {
            end = mid
        } else if days < D {
            end = mid
        } else {
            start = mid + 1
        }
    }
    return start
}

private func shipDays(_ weights: [Int], _ limit: Int) -> Int {
    var i = 0
    var sum = 0
    var days = 1
    while i < weights.count {
        if sum + weights[i] > limit {
            sum = weights[i]
            days += 1
        } else {
            sum += weights[i]
        }
        i += 1
    }
    return days
}