Letter Combinations of a Phone Number

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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"].

分析：

easy

参考代码：

http://www.programcreek.com/2014/04/leetcode-letter-combinations-of-a-phone-number-java/

public class Solution {    public List<String> letterCombinations(String digits) {    HashMap<Integer, String> map = new HashMap<Integer, String>();    map.put(2, "abc");    map.put(3, "def");    map.put(4, "ghi");    map.put(5, "jkl");    map.put(6, "mno");    map.put(7, "pqrs");    map.put(8, "tuv");    map.put(9, "wxyz");    map.put(0, "");     ArrayList<String> result = new ArrayList<String>();     if(digits == null || digits.length() == 0)        return result;     ArrayList<Character> temp = new ArrayList<Character>();    getString(digits, temp, result, map);     return result;} public void getString(String digits, ArrayList<Character> temp, ArrayList<String> result,  HashMap<Integer, String> map){    if(digits.length() == 0){        char[] arr = new char[temp.size()];        for(int i=0; i<temp.size(); i++){            arr[i] = temp.get(i);        }        result.add(String.valueOf(arr));        return;    }     Integer curr = Integer.valueOf(digits.substring(0,1));    String letters = map.get(curr);    for(int i=0; i<letters.length(); i++){        temp.add(letters.charAt(i));        getString(digits.substring(1), temp, result, map);        temp.remove(temp.size()-1);    }  }}

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