LeetCode算法题-17. 电话号码的字母组合（Swift）

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number
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给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例:

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
说明:
尽管上面的答案是按字典序排列的，但是你可以任意选择答案输出的顺序。

方法

 func letterCombinations(_ digits: String) -> [String] {
           
        let dic = [
            "0":[],
            "1":[],
            "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 result = [String]() //存放结果的值
        
        for character in digits {
            let currentArray = dic[String(character)]!//当前对应的键盘上的单词
            
            if result.count == 0 {
                //如果此时结果里面还没值，将当前对应的单词数组加入结果
                for chara in currentArray {
                    result.append(chara)
                }
            }else {
                //1、如果结果有值，遍历结果，取出一个值后，将其拿出。
                //2、遍历当前键盘对应的单词数组，将其拼在第一步拿出的字符串后面，并将这个拼在结果里面。
                let currentResult = result
                
                for i in 0..<currentResult.count {
                    result.remove(at: 0)
                    let resultStr = currentResult[i]
                    for chara in currentArray {
                        let lastChara = resultStr + chara
                        result.append(lastChara)
                    }
                }
            }
        }
        return result
    }