Java ArrayList to print all possible words from phone digits
Last Updated :
15 Sep, 2022
Given a keypad of a mobile, and keys that need to be pressed, the task is to print all the words which are possible to generate by pressing these numbers.
Examples:
Input: str = "12"
Output: [ad, bd, cd, ae, be, ce, af, bf, cf]
Explanation: The characters that can be formed
by pressing 1 is a, b, c and by pressing 2 characters
d, e, f can be formed.
So all the words will be a combination where first
character belongs to a, b, c and 2nd character belongs
to d, e, f
Input: str = "4"
Output: [j, k, l]
Explanation: The characters that can be formed
by pressing 4 is j, k, l
Method 1: Another approach is discussed here Print all possible words from phone digits
Method 2:
Approach: The approach is slightly different from the approach in the other article. Suppose there are n keys which are pressed (a1 a2 a3 ..an). Find all the words that can be formed using (a2 a3 ..an). Suppose 3 characters can be generated by pressing a1 then for every character concatenate the character before all the words and insert them to the list.
For Example:
If the keypress is 12
The characters that can be formed by pressing 1 is a, b, c and by pressing 2 characters d, e, f can be formed.
So all the words that can be formed using 2 are [d, e, f]
So now concatenate ‘a’ with all words returned so, the list is [ad, ae, af] similarly concatenate b and c. So the list becomes [ad, ae, af, bd, be, bf, cd, ce, cf].
Algorithm:
- Write a recursive function that accepts key press string and returns all the words that can be formed in an Array list.
- If the length of the given string is 0 then return Arraylist containing empty string.
- Else recursively call the function with a string except the first character of original string, i.e string containing all the characters from index 1 to n-1. and store the arraylist returned, list and create a new arraylist ans
- Get the character set of the first character of original string, CSet
- For every word of the list run a loop through the Cset and concatenate the character of Cset infront of the word of list and insert them in the ans arraylist.
- Return the arraylist, ans.
Implementation:
Java
import java.util.ArrayList;
public class GFG {
static final String codes[]
= { " ", "abc", "def",
"ghi", "jkl", "mno",
"pqr", "stu", "vwx",
"yz" };
public static ArrayList<String> printKeyWords(String str)
{
if (str.length() == 0) {
ArrayList<String> baseRes = new ArrayList<>();
baseRes.add("");
return baseRes;
}
char ch = str.charAt(0);
String restStr = str.substring(1);
ArrayList<String> prevRes = printKeyWords(restStr);
ArrayList<String> Res = new ArrayList<>();
String code = codes[ch - '0'];
for (String val : prevRes) {
for (int i = 0; i < code.length(); i++) {
Res.add(code.charAt(i) + val);
}
}
return Res;
}
public static void main(String[] args)
{
String str = "23";
System.out.println(printKeyWords(str));
}
}
|
Output
[dg, eg, fg, dh, eh, fh, di, ei, fi]
Complexity Analysis:
- Time Complexity: O(3n).
Though the recursive function runs n times. But the size of the arraylist grows exponentially. So there will be around 3n elements in the arraylist. Therefore, traversing them will take 3n time.
- Space Complexity: O(3n).
The space required to store all words is O(3n). As there will be around 3n words in the output.
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