LeetCode刷题笔记第17题：电话号码的字母组合

LeetCode刷题笔记第17题：电话号码的字母组合

题目：

想法：

先构建手机号码的字典，利用回溯的思想，组合数字对应的字母，代码如下：

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        phone = {"2": ["a", "b", "c"],
                 "3": ["d", "e", "f"],
                 "4": ["g", "h", "i"],
                 "5": ["j", "k", "l"],
                 "6": ["m", "n", "o"],
                 "7": ["p", "q", "r", "s"],
                 "8": ["t", "u", "v"],
                 "9": ["w", "x", "y", "z"]}
        
        def backtrack(now, next_digits):
            if not next_digits:
                ans.append(now)
            else:
                for d in phone[next_digits[0]]:
                    backtrack(now + d, next_digits[1:])
        
        ans = []
        if digits:
            backtrack("", digits)
        return ans

时间复杂度为：$O(4^n)$，其中n为digits的长度
空间复杂度为：$O(n)$，回溯时函数递归的深度