# Print all Possible Decodings of a given Digit Sequence

Given the numeric string str, where 1 represents ‘a’, 2 represents ‘b’, …, 26 represents ‘z’, the task is to print all possible alphabetical strings that can be obtained from str.

Examples:

Input: str = “1123”
Output:
aabc
kbc
alc
aaw
kw
Explanation:
The given string can be splitted as:
1) “1123” = “1” + “1” + “2” + “3” = aabc
2) “1123” = “11” + “2” + “3” = kbc
3) “1123” = “1” + “12” + “3” = alc
4) “1123” = “1” + “1” + “23” = aaw
5) “1123” = “11” + “23” = aabc
Input: str = “56”
Output:
ef
Explanation:
The given string can be splitted as:
1) “56” = “5” + “6” = ef

Approach: It can be observed that every single character represents an alphabet apart from 0. This problem is recursive and can be broken into sub-problems. The terminating condition will be when the passed string is empty. Below are the steps to solve the problem:

1. Create a helper function getChar() that returns the corresponding alphabet of the given numeric character.
2. Create a recursive function that takes the input as a string and returns an array of the desired answer of every character extracted.
3. The base case is when the input string is empty. Return an array of length one containing an empty string for this case.
4. Extract every single character by using the helper function and append it to the empty string first and store it in an array, say output1.
5. At the same time check if the length of the characters is greater than or equal to two and also check if the two characters extracted lies in the range of alphabets. Now, store the corresponding character in an array, say output2.
6. Then create a final output array whose length will be the sum of the length of output1 and output2 and store their answers and return it.
7. Repeat the above steps for every single or pair of adjacent characters extracted. Now, return the final array obtained.
8. Traverse the array and for each string, generate corresponding strings of lowercase alphabets and print them.

Below is the implementation of the above approach:

## Java

 `// Java program for the above approach`   `import` `java.io.*;`   `class` `GFG {`   `    ``// Function to check if all the` `    ``// characters are lowercase or not` `    ``public` `static` `boolean` `    ``nonLower(String s)` `    ``{` `        ``// Traverse the string` `        ``for` `(``int` `i = ``0``; i < s.length(); i++) {`   `            ``// If any character is not` `            ``// found to be in lowerCase` `            ``if` `(!Character` `                     ``.isLowerCase(s.charAt(i))) {` `                ``return` `true``;` `            ``}` `        ``}`   `        ``return` `false``;` `    ``}`   `    ``// Function to print the decodings` `    ``public` `static` `void` `    ``printCodes(String output[])` `    ``{` `        ``for` `(``int` `i = ``0``; i < output.length; i++) {`   `            ``// If all characters are not` `            ``// in lowercase` `            ``if` `(nonLower(output[i]))` `                ``continue``;` `            ``System.out.println(output[i]);` `        ``}` `    ``}`   `    ``// Fuction to return the character` `    ``// corresponding to given integer` `    ``public` `static` `char` `getChar(``int` `n)` `    ``{` `        ``return` `(``char``)(n + ``96``);` `    ``}`   `    ``// Function to return the decodings` `    ``public` `static` `String[] getCode(` `        ``String str)` `    ``{` `        ``// Base case` `        ``if` `(str.length() == ``0``) {`   `            ``String ans[] = { ``""` `};` `            ``return` `ans;` `        ``}`   `        ``// Recursive call` `        ``String output1[]` `            ``= getCode(str.substring(``1``));`   `        ``// Stores the characters of` `        ``// two digit numbers` `        ``String output2[] = ``new` `String[``0``];`   `        ``// Extract first digit and` `        ``// first two digits` `        ``int` `firstDigit` `            ``= (str.charAt(``0``) - ``'0'``);` `        ``int` `firstTwoDigit = ``0``;`   `        ``if` `(str.length() >= ``2``) {`   `            ``firstTwoDigit` `                ``= (str.charAt(``0``) - ``'0'``) * ``10` `                  ``+ (str.charAt(``1``) - ``'0'``);`   `            ``// Check if it lies in the` `            ``// range of alphabets` `            ``if` `(firstTwoDigit >= ``10` `                ``&& firstTwoDigit <= ``26``) {`   `                ``// Next recursive call` `                ``output2` `                    ``= getCode(str.substring(``2``));` `            ``}` `        ``}`   `        ``// Combine both the output in a` `        ``// single final output array` `        ``String output[]` `            ``= ``new` `String[output1.length` `                         ``+ output2.length];`   `        ``// Index of final output array` `        ``int` `k = ``0``;`   `        ``// Store the elements of output1` `        ``// in final output array` `        ``for` `(``int` `i = ``0``; i < output1.length; i++) {`   `            ``char` `ch = getChar(firstDigit);`   `            ``output[i] = ch + output1[i];` `            ``k++;` `        ``}`   `        ``// Store the elements of output2` `        ``// in final output array` `        ``for` `(``int` `i = ``0``; i < output2.length; i++) {`   `            ``char` `ch = getChar(firstTwoDigit);`   `            ``output[k] = ch + output2[i];` `            ``k++;` `        ``}`   `        ``// Result the result` `        ``return` `output;` `    ``}`   `    ``// Driver Code` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``String input = ``"101"``;`   `        ``// Function call` `        ``String output[] = getCode(input);`   `        ``// Print function call` `        ``printCodes(output);` `    ``}` `}`

## C#

 `// C# program for ` `// the above approach` `using` `System;` `class` `GFG{`   `// Function to check if all the` `// characters are lowercase or not` `public` `static` `bool` `nonLower(String s)` `{` `  ``// Traverse the string` `  ``for` `(``int` `i = 0; i < s.Length; i++) ` `  ``{` `    ``// If any character is not` `    ``// found to be in lowerCase` `    ``if` `(!``char``.IsLower(s[i])) ` `    ``{` `      ``return` `true``;` `    ``}` `  ``}` `  ``return` `false``;` `}`   `// Function to print the decodings` `public` `static` `void` `printCodes(String []output)` `{` `  ``for` `(``int` `i = 0; i < output.Length; i++) ` `  ``{` `    ``// If all characters are not` `    ``// in lowercase` `    ``if` `(nonLower(output[i]))` `      ``continue``;` `    ``Console.WriteLine(output[i]);` `  ``}` `}`   `// Fuction to return the character` `// corresponding to given integer` `public` `static` `char` `getChar(``int` `n)` `{` `  ``return` `(``char``)(n + 96);` `}`   `// Function to return the decodings` `public` `static` `String[] getCode(String str)` `{` `  ``// Base case` `  ``if` `(str.Length == 0) ` `  ``{` `    ``String []ans = { ``""` `};` `    ``return` `ans;` `  ``}`   `  ``// Recursive call` `  ``String []output1 = getCode(str.Substring(1));`   `  ``// Stores the characters of` `  ``// two digit numbers` `  ``String []output2 = ``new` `String;`   `  ``// Extract first digit and` `  ``// first two digits` `  ``int` `firstDigit = (str - ``'0'``);` `  ``int` `firstTwoDigit = 0;`   `  ``if` `(str.Length >= 2) ` `  ``{` `    ``firstTwoDigit = (str - ``'0'``) * 10 + ` `                    ``(str - ``'0'``);`   `    ``// Check if it lies in the` `    ``// range of alphabets` `    ``if` `(firstTwoDigit >= 10 && ` `        ``firstTwoDigit <= 26) ` `    ``{` `      ``// Next recursive call` `      ``output2 = getCode(str.Substring(2));` `    ``}` `  ``}` `  `  `  ``// Combine both the output in a` `  ``// single readonly output array` `  ``String []output = ``new` `String[output1.Length + ` `                               ``output2.Length];`   `  ``// Index of readonly output array` `  ``int` `k = 0;`   `  ``// Store the elements of output1` `  ``// in readonly output array` `  ``for` `(``int` `i = 0; i < output1.Length; i++) ` `  ``{` `    ``char` `ch = getChar(firstDigit);` `    ``output[i] = ch + output1[i];` `    ``k++;` `  ``}`   `  ``// Store the elements of output2` `  ``// in readonly output array` `  ``for` `(``int` `i = 0; i < output2.Length; i++) ` `  ``{` `    ``char` `ch = getChar(firstTwoDigit);    ` `    ``output[k] = ch + output2[i];` `    ``k++;` `  ``}`   `  ``// Result the result` `  ``return` `output;` `}`   `// Driver Code` `public` `static` `void` `Main(String[] args)` `{` `  ``String input = ``"101"``;`   `  ``// Function call` `  ``String []output = getCode(input);`   `  ``// Print function call` `  ``printCodes(output);` `}` `}`   `// This code is contributed by Rajput-Ji`

Output:

```ja

```

Time Complexity: O(2N)
Space Complexity:O(N)

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