17. Letter Combinations of a Phone Number

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

1. Use DFS:

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if (digits.empty()) return {};
        vector<vector<char>> d;
        d.push_back({' '});
        d.push_back({});
        d.push_back({'a','b','c'});
        d.push_back({'d','e','f'});
        d.push_back({'g','h','i'});
        d.push_back({'j','k','l'});
        d.push_back({'m','n','o'});
        d.push_back({'p','q','r','s'});
        d.push_back({'t','u','v'});
        d.push_back({'w','x','y','z'});
        vector<string> result;
        string curr = "";
        DFS(digits, result, curr, d, 0);
        return result;
    }
private:
    void DFS(string digits, vector<string>& result, string& curr, vector<vector<char>> d, int len){
        if(len==digits.length()){
            result.push_back(curr);
            return;
        }
        int s = digits[len] - '0';
        for(auto a : d[s]){
            curr.push_back(a);
            DFS(digits, result, curr, d, len + 1);
            curr.pop_back();
        }
    }
};