69. Letter Combinations of a Phone Number

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.

思路： hash && dfs.

void dfs(vector<string>& out, string ans, int curDepth, string& digits, string map[10] ){
    if(curDepth == digits.length()){
        out.push_back(ans);
        return;
    }
    for(int i = 0; i < map[digits[curDepth]-'0'].length(); ++i){
        ans.push_back(map[digits[curDepth]-'0'][i]);
        dfs(out, ans, curDepth+1, digits, map);
        ans.pop_back();
    }
}

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> out;
        string ans;
        string map[10] = {"", "", "abc","def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        dfs(out, ans, 0, digits, map);
        return out;
    }
};