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LetterPhoneNumber.java
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package backtracking;
import java.util.ArrayList;
import java.util.List;
/**
* Created by gouthamvidyapradhan on 09/03/2017.
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.
1 2(abc) 3(def)
4(ghi) 5(jkl) 6(mno)
7(pqrs) 8(tuv) 9(wxyz)
Input:Digit string "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.
*/
public class LetterPhoneNumber
{
private String[] NUMBER_ALPHA = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
/**
* Main method
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception
{
List<String> result = new LetterPhoneNumber().letterCombinations("23");
result.forEach(System.out::println);
}
private List<String> letterCombinations(String digits)
{
if(digits == null || digits.isEmpty() || digits.contains("1") || digits.contains("0")) return new ArrayList<>();
List<String> prev = new ArrayList<>();
prev.add("");
for(int i = digits.length() - 1; i >= 0; i --)
{
String str = NUMBER_ALPHA[Integer.parseInt(String.valueOf(digits.charAt(i)))];
List<String> newList = new ArrayList<>();
for(int j = 0, l = str.length(); j < l; j ++)
{
for(String s : prev)
{
s = str.charAt(j) + s;
newList.add(s);
}
}
prev = newList;
}
return prev;
}
}