```
vector<string> letterCombinations(string digits) {
vector<string> result;
if(digits.empty()) return vector<string>();
static const vector<string> v = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
result.push_back(""); // add a seed for the initial case
for(int i = 0 ; i < digits.size(); ++i) {
int num = digits[i]-'0';
if(num < 0 || num > 9) break;
const string& candidate = v[num];
if(candidate.empty()) continue;
vector<string> tmp;
for(int j = 0 ; j < candidate.size() ; ++j) {
for(int k = 0 ; k < result.size() ; ++k) {
tmp.push_back(result[k] + candidate[j]);
}
}
result.swap(tmp);
}
return result;
}
```

Simple and efficient iterative solution.

Explanation with sample input "123"

Initial state:

- result = {""}

Stage 1 for number "1":

- result has {""}
- candiate is "abc"
- generate three strings "" + "a", ""+"b", ""+"c" and put into tmp,

tmp = {"a", "b","c"} - swap result and tmp (swap does not take memory copy)
- Now result has {"a", "b", "c"}

Stage 2 for number "2":

- result has {"a", "b", "c"}
- candidate is "def"
- generate nine strings and put into tmp,

"a" + "d", "a"+"e", "a"+"f",

"b" + "d", "b"+"e", "b"+"f",

"c" + "d", "c"+"e", "c"+"f" - so tmp has {"ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf" }
- swap result and tmp
- Now result has {"ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf" }

Stage 3 for number "3":

- result has {"ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf" }
- candidate is "ghi"
- generate 27 strings and put into tmp,
- add "g" for each of "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"
- add "h" for each of "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"
- add "h" for each of "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"
- so, tmp has

{"adg", "aeg", "afg", "bdg", "beg", "bfg", "cdg", "ceg", "cfg"

"adh", "aeh", "afh", "bdh", "beh", "bfh", "cdh", "ceh", "cfh"

"adi", "aei", "afi", "bdi", "bei", "bfi", "cdi", "cei", "cfi" } - swap result and tmp
- Now result has

{"adg", "aeg", "afg", "bdg", "beg", "bfg", "cdg", "ceg", "cfg"

"adh", "aeh", "afh", "bdh", "beh", "bfh", "cdh", "ceh", "cfh"

"adi", "aei", "afi", "bdi", "bei", "bfi", "cdi", "cei", "cfi" }

Finally, return result.