LeetCode 17 Letter Combinations of a Phone Number

class Solution {
public:
    char a[10][5]={{'\0'},{'\0'},{'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'}};
    vector<string> ans;
    vector<string> letterCombinations(string digits) {
        
        if(digits.length()==0)
            return ans;
        fun(digits,0,"");
       
        return ans;
    }
    
    void fun(string digits,int index,string res)
    {
        if(index==digits.length())
        {
            ans.push_back(res);
            return;
        }
        
        int x = digits[index]-'0';
        
        for(int i=0;i<5;i++)
        {
            if(a[x][i]=='\0')
                break;
            
            fun(digits,index+1,res+a[x][i]);
        }
        
    }
};