17. 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"].

思路:就是设置一个字典，然后回溯的组合就行

代码如下(已通过leetcode)

public class Solution {
   public List<String> letterCombinations(String digits) {
    String[] dicts={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    List<String> list=new ArrayList<String>();
    if(digits==null||digits.length()==0) return list;
    String temp="";
    backtrace(list,digits,0,dicts,temp);
    return list;
   }


private void backtrace(List<String> list, String digits, int start, String[] dicts, String temp) {
// TODO Auto-generated method stub
if(start==digits.length()) {
list.add(temp);
return;
}
for(int i=start;i<digits.length();i++) {
String str=dicts[digits.charAt(i)-'0'];
for(int j=0;j<str.length();j++) {
temp=temp+str.charAt(j);
backtrace(list, digits, start+1, dicts, temp);
temp=temp.substring(0,temp.length()-1);
}
break;
}

}



}