Letter Combinations main function structure.

letter_combinations_2.swift

  func letterCombinations(_ digits: String) -> [String] {
        // Empty check
        guard digits.isEmpty == false else { return [] }
        // Result array
        var output = [String]()
        // recursive call with "" and index : 0
        combinations_dfs_helper(digits,"",0,&output)
        return output
    }