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
+    }
+}