[LeetCode]17. Letter Combinations of a Phone Number

Description

Given a digit string, 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.

Example

Input:Digit string “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.

Discussion

这是一个典型的遍历搜索问题。我们可以采用深度优先搜索的方法。输入给出一串数字，每一个数字给搜索树加一层。

算法的时间复杂度为O(3^n)。

C++ Code

class Solution {public:    vector<string> letterCombinations(string digits) {        string dict[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};        vector<string> answer;        if(digits.length() == 0)        {            return answer;        }        dfs(digits, dict, 0, "", answer);        return answer;    }    void dfs(string digits, string dict[], int level, string prefix, vector<string> &answer)    {        if(level == digits.length())        {            answer.push_back(prefix);            return;        }        int index = digits[level] - '2';        //在字典dict中的下标        //遍历该数字对应的每一种字符情况        for(int i = 0; i < dict[index].size(); i++)        {            dfs(digits, dict, level + 1, prefix + dict[index][i], answer);        }    }};