leetcode 17. Letter Combinations of a Phone Number 谷歌面试题
Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.
Example:
Input: "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.
class Solution {
public static List<String> letterCombinations(String digits) {
List<String> fi= new ArrayList<String>();
if(digits == null || digits.length() == 0){
return fi;
}
generateString(digits, fi , "");
return fi;
}
public static void generateString(String digits, List<String> fi,String result){
if(digits.length() == 0) {
fi.add(result);
return;
}
String head = get_head(digits);
String a = "";
switch (head) {
case "2":
a = "abc";
break;
case "3":
a = "def";
break;
case "4":
a = "ghi";
break;
case "5":
a = "jkl";
break;
case "6":
a = "mno";
break;
case "7":
a = "pqrs";
break;
case "8":
a = "tuv";
break;
case "9":
a = "wxyz";
break;
default:
a = "";
break;
}
for(int i = 0; i < a.length(); i++) {
result = result + a.substring(i,i+1);
generateString(digits.substring(1,digits.length()),fi,result);
result = result.substring(0,result.length()-1);
}
}
public static String get_head(String digits){
if(digits == null || digits == ""){
return null;
}
return digits.substring(0,1);
}
}
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