Leetcode17:电话号码的字母组合——回溯

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if digits=="":
            return []
        tel = {"2":"abc","3":"def","4":"ghi","5":"jkl","6":"mno","7":"pqrs","8":"tuv","9":"wxyz"}
        size = len(digits)
        path = []
        res = []
        def backtracking(i, path, res):
            for letter in tel[digits[i]]:
                path.append(letter)
                if i==size-1:
                    res.append("".join(path))
                    path.pop()
                else:
                    backtracking(i+1, path, res)
                    path.pop()
        backtracking(0, path, res)
        return res