17. 电话号码的字母组合

class Solution {
    let map :Dictionary<Character,Array<Character>> = [
        "2" : ["a","b","c"],
        "3" : ["d","e","f"],
        "4" : ["g","h","i"],
        "5" : ["j","k","l"],
        "6" : ["m","n","o"],
        "7" : ["p","q","r","s"],
        "8" : ["t","u","v"],
        "9" : ["w","x","y","z"]
    ]
    var ans = Array<String>()
    var tempStrArray = Array<Character>()
    
   func letterCombinations(_ digits: String) -> [String] {

        let len = digits.count
        if len == 0 {
            return []
        }
        tempStrArray = Array.init(repeating: " ", count: len)
        dfs(0,Array(digits))
        return ans
        
    }
    
    func dfs(_ index:Int,_ digitsArray:Array<Character>) {
        let len = digitsArray.count
        //说明到达了底部，可以获得一个正确的解
        if index == len   {
            ans.append(String.init(tempStrArray))
            return
        }
        let key = digitsArray[index]
        let array = map[key]!
        //枚举出来这一层
        for c in array {
            tempStrArray[index] = c
            dfs(index + 1,digitsArray)
        }

    }
}

改造版本

  let map :Dictionary<Character,Array<Character>> = [
        "2" : ["a","b","c"],
        "3" : ["d","e","f"],
        "4" : ["g","h","i"],
        "5" : ["j","k","l"],
        "6" : ["m","n","o"],
        "7" : ["p","q","r","s"],
        "8" : ["t","u","v"],
        "9" : ["w","x","y","z"]
    ]
   
    
    func letterCombinations(_ digits: String) -> [String] {

        let len = digits.count
        if len == 0 {
            return []
        }
        var ans = Array<String>()
        var tempStrArray : Array<Character> = Array.init(repeating: " ", count: len)
        dfs(0,Array(digits),&ans,&tempStrArray)
        return ans
        
    }
    
    func dfs(_ index:Int,_ digitsArray:Array<Character>,  _ ans:inout Array<String>,  _ tempStrArray: inout Array<Character>) {
        let len = digitsArray.count
        //说明到达了底部，可以获得一个正确的解
        if index == len   {
            ans.append(String.init(tempStrArray))
            return
        }
        let key = digitsArray[index]
        let array = map[key]!
        //枚举出来这一层
        for c in array {
            tempStrArray[index] = c
            dfs(index + 1,digitsArray,&ans,&tempStrArray)
        }

    }