题目:
题解:
class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> combinations;
if (digits.empty()) {
return combinations;
}
unordered_map<char, string> phoneMap{
{'2', "abc"},
{'3', "def"},
{'4', "ghi"},
{'5', "jkl"},
{'6', "mno"},
{'7', "pqrs"},
{'8', "tuv"},
{'9', "wxyz"}
};
string combination;
backtrack(combinations, phoneMap, digits, 0, combination);
return combinations;
}
void backtrack(vector<string>& combinations, const unordered_map<char, string>& phoneMap, const string& digits, int index, string& combination) {
if (index == digits.length()) {
combinations.push_back(combination);
} else {
char digit = digits[index];
const string& letters = phoneMap.at(digit);
for (const char& letter: letters) {
combination.push_back(letter);
backtrack(combinations, phoneMap, digits, index + 1, combination);
combination.pop_back();
}
}
}
};