17. Letter Combinations of a Phone Number - cocoder39/coco39_LC GitHub Wiki

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if (digits.empty()) {
            return {};
        }
        
        vector<string> encode = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        vector<string> res;
        string cur;
        helper(res, cur, encode, digits, 0);
        return res;
    }
private:
    void helper(vector<string>& res, string& cur, vector<string>& encode, string& digits, int start) {
        if (start == digits.length()) {
            res.push_back(cur);
            return;
        }
        
        int num = digits[start] - '0';
        for (auto c : encode[num]) {
            cur.push_back(c);
            helper(res, cur, encode, digits, start + 1);
            cur.pop_back();
        }
    }
};

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if (digits.empty()) return {};
        
        queue<string> qu;
        qu.push("");
        for (int i = 0; i < digits.length(); i++) {
            string& encode = encodes[digits[i] - '0'];
            if (encode.empty()) return {};
            int sz = qu.size();
            for (int i = 0; i < sz; i++) {
                string& str = qu.front();
                qu.pop();
                for (auto c : encode) {
                    str.push_back(c);
                    qu.push(str);
                    str.pop_back();
                }
            }
        }
        
        vector<string> res;
        while (! qu.empty()) {
            res.push_back(qu.front());
            qu.pop();
        }
        return res;
    }
private:
    vector<string> encodes{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
};