find extension

func findExtension(_ str: String) -> [Int]{
    let arr = Array(str)
    var start = 0
    var runner = 0
    var res = [Int]()
    while runner < arr.count{
        while runner < arr.count && arr[runner] == arr[start]{
            runner += 1
        }
        if runner - start >= 3 {
            res.append(start)
        }
        start = runner
        runner += 1
    }
    return res
}


let str = "aaaafffddd"
print(findExtension(str))