151. Reverse Words in a String

    func reverseWords(_ s: String) -> String {
        let arr = Array(s.trimmingCharacters(in: .whitespaces))
        var runner = arr.count - 1
        var last = arr.count - 1
        var res = [String]()
        while runner >= 0{
            var last = runner
            while runner >= 0 && arr[runner] != " " {
                runner -= 1
            }
            res.append(String(arr[runner + 1 ... last]))
            while runner >= 0 && arr[runner] == " " {
                runner -= 1
            }
        }
        if res.count == 0 {return ""}
        var i = 0
        var ans = res[i]
        i += 1
        while i < res.count {
            ans = ans + " " + res[i]
            i += 1
        }
        return ans
    }



//用系统API
    func reverseWords(_ s: String) -> String {
        let parts = s.split(separator: " ").reversed()
        return parts.joined(separator: " ")
    }