Possible Words From Phone Digits

recursion/Problems/PossibleWordsFromPhoneDigits.java

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+/*Given a keypad as shown in the diagram, and an N digit number which is represented by array a[ ], the task is to list all words which are possible by pressing these numbers.
+
+
+Example 1:
+
+Input: N = 3, a[] = {2, 3, 4}
+Output:
+adg adh adi aeg aeh aei afg afh afi 
+bdg bdh bdi beg beh bei bfg bfh bfi 
+cdg cdh cdi ceg ceh cei cfg cfh cfi 
+Explanation: When we press 2,3,4 then 
+adg, adh, adi, ... cfi are the list of 
+possible words.
+Example 2:
+
+Input: N = 3, a[] = {3, 4, 5}
+Output:
+dgj dgk dgl dhj dhk dhl dij dik dil 
+egj egk egl ehj ehk ehl eij eik eil 
+fgj fgk fgl fhj fhk fhl fij fik fil
+Explanation: When we press 3,4,5 then 
+dgj, dgk, dgl, ... fil are the list of 
+possible words.
+Your Task:
+You don't need to read input or print anything. You just need to complete the function possibleWords() that takes an array a[ ] and N as input parameters and returns an array of all the possible words in lexicographical increasing order. 
+
+Expected Time Complexity: O(4N * N).
+Expected Auxiliary Space: O(N).
+
+Constraints:
+1 ≤ N ≤ 10
+2 ≤ a[i] ≤ 9 */
+
+
+
+
+
+package recursion.Problems;
+
+import java.util.ArrayList;
+
+public class PossibleWordsFromPhoneDigits {
+    static ArrayList <String> possibleWords(int a[], int N)
+    {
+        // your code here  
+             ArrayList<String> res = fetchCombinations(a,N,0);
+        return res;
+    }
+
+    private static ArrayList<String> fetchCombinations(int[] a, int n, int current)
+    {
+        if(current == n)
+        {
+            ArrayList<String> res = new ArrayList<>();
+            res.add("");
+            return res;
+        }
+
+        int currentNum = a[current];
+        String currentString = "";
+
+        switch(currentNum)
+        {
+            case 1 : currentString = ""; break;
+            case 2 : currentString = "abc"; break;
+            case 3 : currentString = "def"; break;
+            case 4 : currentString = "ghi"; break;
+            case 5 : currentString = "jkl"; break;
+            case 6 : currentString = "mno"; break;
+            case 7 : currentString = "pqrs"; break;
+            case 8 : currentString = "tuv"; break;
+            case 9 : currentString = "wxyz"; break;
+            case 0 : currentString = ""; break;
+            default : currentString = ""; break;
+        }
+
+        ArrayList<String> others = fetchCombinations(a,n,current + 1);
+        ArrayList<String> res = new ArrayList<>();
+
+        for(int i = 0; i<currentString.length(); i++)
+        {
+            String curr = ""+currentString.charAt(i);
+
+            for(String str : others)
+            res.add(curr+str);
+        }
+
+        return res;
+    }
+}