Letter Combinations of a Phone Number解题报告

简单的递归。。。

Code:

vector<string> letterCombinations(string digits) {
		if(digits.size() <= 0)	return vector<string>();
		map<int, string> mp;
		mp[0] = " "; mp[1] = "@"; mp[2] = "abc"; mp[3] = "def";
		mp[4] = "ghi"; mp[5] = "jkl"; mp[6] = "mno"; mp[7] = "pqrs";
		mp[8] = "tuv"; mp[9] = "wxyz";
		return letterCombinations_(digits, 0, mp);
    }
	vector<string> letterCombinations_(string digits, int pos, map<int, string> &mp){
		int digit = digits[pos] - '0';
		string str = mp[digit];
		if(pos == digits.size() - 1){
			vector<string> vstr(str.size());
			for(int i = 0; i < vstr.size(); ++i)	vstr[i] += str[i];
			return vstr;
		}
		vector<string> vstr_ = letterCombinations_(digits, pos + 1, mp);
		vector<string> vstr(vstr_.size() * str.size());
		for(int i = 0; i < str.length(); ++i){
			int size = vstr_.size();
			for(int j = 0; j < size; ++j){
				int idx = j + i * size;
				vstr[idx] += str[i];
				vstr[idx] += vstr_[j];
			}
		}
		return vstr;
	}