LeetCode Solution:Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

Total Accepted: 17652 Total
Submissions: 66854My Submissions

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"].

public class Solution {
    public List<String> letterCombinations(String digits) {
     List<String> result = new ArrayList<String>();
     if (digits == null) {
         return result;
     }

     List<String> findSet = inSet(digits);
     helper(findSet, new String(), 0, result);
     return result;
    }
   List<String> inSet(String digits) {
            String[] keyboard ={" ","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
            List<String> findSet = new ArrayList<String>();
            for (int i = 0; i < digits.length(); i++) {
                findSet.add(keyboard[digits.charAt(i) - '0']);
            }
            return findSet;

   }

    void helper(List<String> findSet,
                String level,
                int levelNum,
                List<String> result) {
        if (level.length() == findSet.size()) {
            result.add(level);
            return;
        }
        for (int i = 0; i < findSet.get(levelNum).length(); i++ ) {
              helper(findSet,level + findSet.get(levelNum).charAt(i)
                     , levelNum + 1, result);
        }
    }
}

做到100题了，Mark一下！