leetcode 17 Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

 
思路:回溯法
 1 class Solution {
 2 private:
 3     // 创建映射map
 4     unordered_map<char, string> num2char {{'2', "abc"}, {'3', "def"}, {'4', "ghi"}, {'5', "jkl"},
 5                                           {'6', "mno"}, {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"}};
 6     
 7     void letterCombinations(vector<string> &res, string &combination, string digits, int index, int len) {
 8         if (index == len) {
 9             res.push_back(combination);
10             return;
11         }
12         //遍历第index位可以选择的字符
13         for (char c : num2char[digits[index]]) {
14             combination.push_back(c);
15             letterCombinations(res, combination, digits, index + 1, len);
16             combination.pop_back();
17         } 
18         /*
19         string current = num2char[digits[index]];
20         for (int i = 0; i < current.length(); i++) {
21             combination.push_back(current[i]);
22             letterCombinations(res, combination, digits, index + 1, len);
23             combination.pop_back();
24         } */
25     }
26 public:
27     vector<string> letterCombinations(string digits) {
28     
29         int len = digits.length();
30         vector<string> res;
31         if (len == 0) {
32             return res;
33         }
34         
35         string combination;
36         letterCombinations(res, combination, digits, 0, len);
37         return res;
38     }
39 };

 

posted @ 2019-08-29 10:20  琴影  阅读(170)  评论(0编辑  收藏  举报