一、题目描述：

题目内容
题目示例
题目解析
- 本题要求给定2～9数字组成的字符串
- 要求对数字字串包含的字母进行组合
- 返回的组合字母顺序可以是任意的。

二、思路分析：

我们拿到本题，给出的2～9数字键对应字母的电话按键，进行组合。

首先，我们会想到使用一个字典，来存储2～9数字，所对应的字母。

Map = {
  "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"]
    }

那怎么将字母进行组合呢？又是眼睛会，手不会的题。

我们先来根据示列1，“23”-〉["ad","ae","af","bd","be","bf","cd","ce","cf"]推理一下：

如上图所示，主要思路如下：

首先，我们需要使用result存储组合字母，currmap存储获取Map数字映射的字母列表
每当取第一个数字字母序列与第二个数字字母序列组合字母通过tmp列表传递给result存储
result已存在的组合字母继续与第三个数字字母序列进行组合tmp列表传递给result
直到遍历完整个digits字符串，返回 result 列表

以上思路，使用的动态规划方法。

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        Map = {
        "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"]
        }
        result = []
        currmap = []
        for i in range(len(digits)):

            curnum = digits[i]
            currmap = Map[curnum]

            if i == 0:

                result = currmap
                continue
            
            tmp = []
            
            for x in range(len(result)):

                for y in range(len(currmap)):

                    tmp.append(result[x]+currmap[y])
                
            result = tmp

        return result

三、总结：

我们提交代码，AC记录如下：时间复杂度O(n^2),空间复杂度O(n)

本题难点在于组合细节过程要想清楚。

以上是本期内容，欢迎大佬们点赞评论，下期见～～～

【LeetCode】17. 电话号码的字母组合

一、题目描述：

题目内容

题目示例

题目解析

二、思路分析：

三、总结：