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.

Example 1:

Input: digits = "23"

Example 2:

Input: digits = ""
Output: []

Example 3:

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

c++ implementation:

vector<string> letterCombinations(string digits)
{
vector<string> result ;
if (digits.length() == 0) return result;

string dict[10]= {"0", "1", "abc", "def", "ghi", "jkl",
"mno", "pqrs", "tuv","wxyz"};

backtrack(result, digits,"", dict);
return result;
}

void backtrack(vector<string>&result,string digits, string cur,
string dict[])
{
if (cur.length() == digits.length())
{
result.push_back(cur);
return;
}

int index=cur.length();

string letter=dict[digits[index]-'0'];

for (int i=0;i<letter.length();i++)
{

backtrack(result, digits, cur+letter[i] , dict);
}

}

darkmode