來源：力扣（LeetCode）
鏈接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number
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給定一個(gè)僅包含數(shù)字 2-9 的字符串，返回所有它能表示的字母組合。

給出數(shù)字到字母的映射如下（與電話按鍵相同）。注意 1 不對應(yīng)任何字母。

image.png

示例:

輸入："23"
輸出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
說明:
盡管上面的答案是按字典序排列的，但是你可以任意選擇答案輸出的順序。

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]() //存放結(jié)果的值
        
        for character in digits {
            let currentArray = dic[String(character)]!//當(dāng)前對應(yīng)的鍵盤上的單詞
            
            if result.count == 0 {
                //如果此時(shí)結(jié)果里面還沒值，將當(dāng)前對應(yīng)的單詞數(shù)組加入結(jié)果
                for chara in currentArray {
                    result.append(chara)
                }
            }else {
                //1、如果結(jié)果有值，遍歷結(jié)果，取出一個(gè)值后，將其拿出。
                //2、遍歷當(dāng)前鍵盤對應(yīng)的單詞數(shù)組，將其拼在第一步拿出的字符串后面，并將這個(gè)拼在結(jié)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
    }