题目

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。

17_telephone_keypad.png

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。

Swift解法

class Solution {
        var dictionary: [Character: [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 results = [String]()
        func letterCombinations(_ digits: String) -> [String] {
            var array = digits.map { dictionary[$0] ?? [] }
            let counts = array.map({$0.count})
            if counts.contains(0) || counts.isEmpty {
                return []
            }
            var flags = [Int](repeating: 0, count: array.count)
            var temperate = ""
            var answers = [String]()
            var affectedFields = [Int]()
            loop: while true {
                if temperate.isEmpty {
                    for j in 0..<flags.count {
                        temperate.append(array[j][flags[j]])
                    }
                } else {
                    for i in affectedFields {
                        temperate.replaceSubrange(temperate.index(temperate.startIndex, offsetBy: i)..<temperate.index(temperate.startIndex, offsetBy: i + 1), with: String(array[i][flags[i]]))
                    }
                    affectedFields.removeAll()
                }
                answers.append(temperate)
                var j = flags.count - 1
                while true {
                    flags[j] += 1
                    affectedFields.append(j)
                    if flags[j] < counts[j] {
                        break
                    } else {
                        flags[j] = 0
                        j -= 1
                        if j < 0 {
                            break loop
                        }
                    }
                    
                }
                
            }
            return answers
        }
    }

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number