[LeetCode17]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.

Analysis:

The idea is to use recursion. DFS slove it.

Java

public class LetterCombinationPhoneNum {ArrayList<String> res;StringBuffer seq;public void getletterComba(String digits, int level, String[] trans){if(level == digits.length()){res.add(seq.toString());return;}int digit = digits.charAt(level)-48;for(int i=0;i<trans[digit].length();i++){seq.append(trans[digit].charAt(i));getletterComba(digits, level+1, trans);seq.deleteCharAt(level);}}public List<String> letterCombinations(String digits) {res = new ArrayList<>();seq = new StringBuffer();        String []trans = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        getletterComba(digits, 0, trans);        return res;    }}

c++

public:    void getLetterCombination(string &seq,                          string &digits,                          int level,                          vector<string> &result,string trans[]){    if(level == digits.size()){        result.push_back(seq);        return;    }    int group = digits[level] - 48;    for(int i=0; i<trans[group].size(); i++){        seq.push_back(trans[group][i]);        getLetterCombination(seq,digits,level+1,result,trans);        seq.resize(seq.size()-1);    }}vector<string> letterCombinations(string digits) {    string trans[] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};    vector<string> result;    string seq;    getLetterCombination(seq,digits,0,result,trans);    return result;}