# Letter Combinations of a Phone Number Problem

## Description

LeetCode Problem 17.

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

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

``````1
2
3
4
5
6
7
8
2: abc
3: def
4: ghi
5: jkl
6: mno
7: pqrs
8: tuv
9: wxyz
``````

Example 1:

``````1
2
Input: digits = "23"
``````

Example 2:

``````1
2
Input: digits = ""
Output: []
``````

Example 3:

``````1
2
Input: digits = "2"
Output: ["a","b","c"]
``````

Constraints:

• 0 <= digits.length <= 4
• digits[i] is a digit in the range [‘2’, ‘9’].

## Sample C++ Code

``````1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
class Solution {
public:
vector<string> ans;
vector<string> mapping = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

void dfs(string digits, string letters) {
if (digits.size() == 0) {
return;
}
int dig = digits[0] - '0';
for (int i = 0; i < mapping[dig].size(); i ++) {
letters += mapping[dig][i];
if (digits.size() == 1)
ans.push_back(letters);
else
dfs(digits.substr(1), letters);
letters.erase(letters.end()-1);
}
return;
}

vector<string> letterCombinations(string digits) {
dfs(digits, "");
return ans;
}
};
``````