Leetcode #900: RLE Iterator

class RLEIterator {
    private var ar = [Int]()
    private var num = [Int]()
    private var current = 0
    private var index = 0
    
    init(_ A: [Int]) {
        for i in stride(from: 0, to: A.count, by: 2) {
            if i == 0 {
                ar.append(A[i])
            } else {
                ar.append(A[i] + ar.last!)
            }
            num.append(A[i+1])
        }
    }
    
    func next(_ n: Int) -> Int {
        current += n
        for i in stride(from: index, to: ar.count, by: 1) {
            if ar[i] >= current {
                index = i
                return num[i]
            }
        }
        return -1
    }
}