Swift: 17. Letter Combinations of a Phone Number

swift/0017-letter-combinations-of-a-phone-number.swift

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+class Solution {
+ private let numberLetters: [Character: String] = [
+ "2": "abc",
+ "3": "def",
+ "4": "ghi",
+ "5": "jkl",
+ "6": "mno",
+ "7": "pqrs",
+ "8": "tuv",
+ "9": "wxyz"
+ ]
+ func letterCombinations(_ digits: String) -> [String] {
+ guard !digits.isEmpty else { return [] }
+ var result: [String] = []
+ var digits = digits.map { $0 }
+ func generateCombinations(_ index: Int, _ path: String) {
+ guard index < digits.count else {
+ result.append(path)
+ return
+ }
+ let digit = digits[index]
+ if let letters = numberLetters[digit] {
+ for letter in letters {
+ generateCombinations(index + 1, path + "\(letter)")
+ }
+ }
+ }
+ generateCombinations(0, "")
+ return result
+ }
+}