Problem Description
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
Solution
We populate an Array of List of Characters in which every digit points to a List
Instead of using array of list of characters, we can also use a HashMap:
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Map<Character, String> letters = Map.of(
'2', "abc", '3', "def", '4', "ghi", '5', "jkl",
'6', "mno", '7', "pqrs", '8', "tuv", '9', "wxyz");
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class Solution {
public List<String> letterCombinations(String digits) {
//edge case
if(digits.equals("")) return new ArrayList<String>();
//convert dial pad as an Array of List of characters.
//index 0, index 1 -> null
//index 2 -> [a, b, c]
//index 3 -> [b, c, d]
//...
//index 8 -> [t u v]
//index 9 -> [w, x, y, z]
List<Character>[] nums = createList();
combinationHelper(digits, nums, "", 0);
return ans;
}
List<String> ans = new ArrayList<>();
public void combinationHelper(String digits, List<Character>[] nums, String current, int idx){
//idx will be tracking the number of digits/characters considered already
//if it is equal to the number of digits in the input, we can include that as one of the combinations
if(idx >= digits.length()){
ans.add(current);
return;
}
//get current digit in the input
int c = Integer.parseInt(digits.charAt(idx)+"");
//get the list of characters it can denote
List<Character> characters = nums[c];
//loop through each possible character
for(int i=0; i<characters.size(); i++){
//append that in the current string
current += characters.get(i);
//recursively call combinationHelper, going to the next digit
combinationHelper(digits, nums, current, idx+1);
//backtrack -> restore the current string by removing the last character which was appended
current = current.substring(0, current.length()-1);
}
}
public List[] createList(){
List<Character>[] nums = new List[10];
List<Character> temp = new ArrayList<>();
temp.add('a');
temp.add('b');
temp.add('c');
nums[2] = temp;
temp = new ArrayList<>();
temp.add('d');
temp.add('e');
temp.add('f');
nums[3] = temp;
temp = new ArrayList<>();
temp.add('g');
temp.add('h');
temp.add('i');
nums[4] = temp;
temp = new ArrayList<>();
temp.add('j');
temp.add('k');
temp.add('l');
nums[5] = temp;
temp = new ArrayList<>();
temp.add('m');
temp.add('n');
temp.add('o');
nums[6] = temp;
temp = new ArrayList<>();
temp.add('p');
temp.add('q');
temp.add('r');
temp.add('s');
nums[7] = temp;
temp = new ArrayList<>();
temp.add('t');
temp.add('u');
temp.add('v');
nums[8] = temp;
temp = new ArrayList<>();
temp.add('w');
temp.add('x');
temp.add('y');
temp.add('z');
nums[9] = temp;
return nums;
}
}