Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.
Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Note: Although the above answer is in lexicographical order, your answer could be in any order you want.
分别遍历追加即可，注意c++中字符串追加字符不能用“” + c, 而只能用string() + c,这一点和java的不同之处。
另外注意c++lambda表达式捕获变量:
[&result] 表示捕获result，并且result按引用传递
[result] 表示捕获result，并且result按值传递
[&] 所有变量按引用传递
[=] 所有变量传值
[] 不捕获任何变量
class Solution {
public:
Solution() {
map['2'] = "abc";
map['3'] = "def";
map['4'] = "ghi";
map['5'] = "jkl";
map['6'] = "mno";
map['7'] = "pqrs";
map['8'] = "tuv";
map['9'] = "wxyz";
}
vector<string> letterCombinations(string digits) {
vector<string> result;
if (digits.size() == 0) {
return result;
}
for (char c : digits) {
result = generate(result, c);
}
return result;
}
vector<string> generate(vector<string> current, char digit) {
vector<string> result;
if (current.size() > 0) {
for (char c : map[digit]) {
for_each(current.begin(), current.end(), [&result, c](string s){result.push_back(s + c);});
}
} else {
for (char c : map[digit])
result.push_back(string() + c);
}
return result;
}
private:
unordered_map<char, string> map;
};