【medium】17. Letter Combinations of a Phone Number 数字组合

class Solution {
public:
    vector<string> ans; // answers
    
    // dfs实现
    void dfs (int x,int l,string str,string digits,char phone[][4]){
        if (x==l){
            ans.push_back(str);
            return;
        }
        int num = digits[x] - ‘0‘;
        for (int i=0;i<4;i++){
            if (phone[num][i]){
                dfs(x+1,l,str+phone[num][i],digits,phone);
            }
        }
    }
    
    vector<string> letterCombinations(string digits) {
        char phone[10][4] = {{" "},{},{‘a‘,‘b‘,‘c‘},{‘d‘,‘e‘,‘f‘},{‘g‘,‘h‘,‘i‘},{‘j‘,‘k‘,‘l‘},{‘m‘,‘n‘,‘o‘},{‘p‘,‘q‘,‘r‘,‘s‘},{‘t‘,‘u‘,‘v‘},{‘w‘,‘x‘,‘y‘,‘z‘}};
        
        if (digits.length() == 0)
            return ans;
        
        // dfs算法！
        dfs(0,digits.length(),"",phone);
            
        return ans;
        
    }
};