solve.cpp

#include <vector>
#include <unordered_map>
#include <string>
#include <algorithm>
#include <iostream>
using namespace std;
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;
};
int main(int argc, char **argv)
{
	Solution solution;
	auto result = solution.letterCombinations("23");
	for_each(result.begin(), result.end(), [](string s){cout << s << endl;});
	return 0;
}