Leetcode Medium 5 Letter Combinations of a Phone Number

• 不用递归 

class Solution:
    def letterCombinations(self, digits):
        dic = {'2':'abc', '3':'def','4':'ghi', '5':'jkl', '6':'mno', '7':'pqrs', '8':'tuv', '9':'wxyz'}
        if not digits:
            return []
        s = str(digits)[::-1]
        l = ['']
        l2 = []
        for i in s:
            for j in dic[i]:
                l2 += [j+x for x in l]
            l = l2
            l2 = []
        return l
            

• 用递归

dic = {'2':'abc', '3':'def','4':'ghi', '5':'jkl', '6':'mno', '7':'pqrs', '8':'tuv', '9':'wxyz'}
class Solution:
    def recursive(self, st, res):
        if not st:
            return res
        if not res:
            res = ['']
        
        res2 = []
        cs = st.pop()
        
        for c in cs:
            res2 += [c+x for x in res]
        return self.recursive(st, res2)
    def letterCombinations(self, digits):
        st = [dic[x] for x in str(digits)]
        return self.recursive(st, [])