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letterCombination.java

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//Given a digit string, return all possible letter combinations that the number could represent. (Check out your cellphone to see the mappings) Input:Digit string "23", Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

//This problem can be solves by a typical DFS algorithm. DFS problems are very similar and can be solved by using a simple recursion. Check out the index page to see other DFS problems.

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);

}