LeetCode #17 Letter Combinations of a Phone Number

LeetCode #17 Letter Combinations of a Phone Number

Question

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Solution

Approach #1

class Solution {
    func letterCombinations(_ digits: String) -> [String] {
        return stringsFromDigits(Array(digits.utf16), index: 0)
    }
    
    func stringsFromDigits(_ digits: [UInt16], index: Int) -> [String] {
        if digits.isEmpty { return [] }
        let current = stringsFromUInt16(digits[index])
        if index == digits.count - 1 { return current }
        let more = stringsFromDigits(digits, index: index + 1)
        var results: [String] = []
        for c in current {
            for m in more {
                results.append(c + m)
            }
        }
        return results
    }
    
    func stringsFromUInt16(_ n: UInt16) -> [String] {
        let zero = "0".utf16.first!
        switch n {
        case zero, zero + 1:
            return [String(utf16CodeUnits: [n], count: 1)]
        case let n where n > zero + 1 && n < zero + 7:
            let a = "a".utf16.first!
            var strs: [String] = []
            for i in UInt16(0)..<UInt16(3) {
                let s = String(utf16CodeUnits: [(n - zero - UInt16(2)) * UInt16(3) + i + a], count: 1)
                strs.append(s)
            }
            return strs
        case zero + 7:
            return ["p", "q", "r", "s"]
        case zero + 8:
            return ["t", "u", "v"]
        case zero + 9:
            return ["w", "x", "y", "z"]
        default:
            return []
        }
    }
}

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