[LeetCode]Letter Combinations of a Phone Number

题目描述:(链接)

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.

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.

解题思路：

递归方式，深度优先搜索

class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> result;
if (digits == "") return result;
vector<string> keyboard {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
string midResult(digits.size(), '\0');
dfs(digits, 0, midResult, keyboard, result);
return result;
}
private:
void dfs(const string &digits, int index, string &midResult, const vector<string> &keyboard, vector<string> &result) {
if (index == digits.size()) {
result.push_back(midResult);
return;
}

char c = digits[index];
for (int i = 0; i < keyboard[c - '0'].size(); ++i) {
midResult[index] = keyboard[c - '0'][i];
dfs(digits, index + 1, midResult, keyboard, result);
}
}
};