topic:
https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/
answer:
public List<String> letterCombinations(String digits) {
List<String> result = new ArrayList<String>();
if (digits.length() == 0) {
return result;
}
Map<Character, String> map = new HashMap<Character, String>() { {
put('2', "abc");
put('3', "def");
put('4', "ghi");
put('5', "jkl");
put('6', "mno");
put('7', "pqrs");
put('8', "tuv");
put('9', "wxyz");
}};
operator(result,map,digits,0,new StringBuffer(""));
return result;
}
//Backtracking
public void operator(List<String> result,Map<Character,String> map, String digits,int index,StringBuffer temp){
if(index == digits.length()){
result.add(temp.toString());
}else{
String s = map.get(digits.charAt(index));
int length = s.length();
for(int i=0;i<length;i++){
temp.append(s.charAt(i));
operator(result,map,digits,index+1,temp);
temp.deleteCharAt(index);
}
}
}