class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> res;
if (digits.empty()) return res;
string dict[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
res.push_back("");
for (int i = 0; i < digits.size(); ++i) {
int n = res.size();
string str = dict[digits[i] - '2'];
for (int j = 0; j < n; ++j) {
string tmp = res.front();
res.erase(res.begin());
for (int k = 0; k < str.size(); ++k) {
res.push_back(tmp + str[k]);
}
}
}
return res;
}
};