Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

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.

题目很简单了，就是按每一位遍历，组合。

vector<string> letterCombinations(string digits) {
vector<string> vec, phone(10);
phone[2] = "abc";
phone[3] = "def";
phone[4] = "ghi";
phone[5] = "jkl";
phone[6] = "mno";
phone[7] = "pqrs";
phone[8] = "tuv";
phone[9] = "wxyz";
letters(phone, 0, "", vec, digits);
return vec;
}

void letters(vector<string> &phone, int num, string str, vector<string> &vec, string digits){
if(num == digits.size()){
vec.push_back(str);
return;
}
int c = digits[num] - '0';
for(int i = 0; i < phone[c].size(); i++){
letters(phone, num+1, str + phone[c][i], vec, digits);
}
}