316. Remove Duplicate Letters

func removeDuplicateLetters(_ s: String) -> String {
        var map = [Character : Int]() // char : count
        var diffMap = [Character : Int]()
        for char in s{
            map[char, default : 0] += 1
        }
        var visited =  Set<Character>()
        var res = [Character]()
        // res.append("0")
        for char in s {
            map[char] =  map[char]! - 1
            if !visited.contains(char) {
                // let last = res.last!
                while res.count > 0 && map[res.last!]! >= 1 && res.last! > char{ //要换
                    let last = res.last!
                    visited.remove(last)
                    res.removeLast()
                }
                visited.insert(char)
                res.append(char)
            }
        }

        // res.removeFirst()
        return String(res)
    }