Letter Combinations of a Phone Number (Leetcode 17)

Problem Link: https://leetcode.com/problems/letter-combinations-of-a-phone-number/

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        
        self.d=[[],[],['a','b','c'],['d','e','f'],['g','h','i'],['j','k','l'],['m','n','o'],['p','q','r','s'],['t','u','v'],['w','x','y','z']]
        
        def fun(digits):
            
            
            if(len(digits)==1):
                num=int(digits[0])
                return self.d[num]
            
            temp=fun(digits[1:])
            number=int(digits[0])
            
            ans=[]
            for i in self.d[number]:
                for j in temp:
                    ans.append(i+j)
                    
            return ans
        
        if(digits==''):
            return []
        return fun(digits)