def letter_combinations(digits):
    if not digits:
        return []
        keypad = {
            "2": "ABC",
            "3": "DEF",
            "4": "GHI,
            "5": "JKL",
            "6": "MNO",
            "7": "PQRS",
            "8": "TUV",
            "9": "WXYZ",
        }
        result = []
        def backtrack(index, current):
            if index == len(digits):
                result.append(current)
                return
            for ch in keypad[digits[index]]:
                backtrack(index + 1, current + ch)
                backtrack(0,"")
                return result
                digits = input("Enter digits: ")
                output = letter_combinations(digits)
                
                print("possible combinations:")
                print(output)