Letter Combinations of a Phone Number 手机的字符表示

Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

class Solution {
public:
    char letter[10][5]={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    int len;
    vector<string> res;
    
    vector<string> letterCombinations(string digits) {
        len=digits.size();
        if(digits=="")
            return res;
        char *c=new char[len+1];
        c[len]='\0';
        res.clear();
        getResult(digits,0,c);
        return res;
    }
    
    void getResult(string num,int cur,char *c)
    {
        if(cur==len)
        {
            res.push_back(c);
            return ;
        }
        char *s=&(letter[ num[cur]-'0' ][0]);
        while(*s!='\0')
        {
            c[cur]=*s;
            getResult(num,cur+1,c);
            s++;
        }
    }
};