Untitled

// Online Java Compiler
// Use this editor to write, compile and run your Java code online
import java.util.*;

class HelloWorld {
    
    public static String reverseDigits(String input){
        ArrayList<Integer> digitsIndices = new ArrayList<Integer>();
        ArrayList<Integer> charIndices = new ArrayList<Integer>();
        
        ArrayList<Character> digits = new ArrayList<Character>();
        ArrayList<Character> characters = new ArrayList<Character>();
        
        // N
        for(int i=0; i < input.length(); i++){
            char c = input.charAt(i);
            if(Character.isDigit(c)){
                digitsIndices.add(i);
                digits.add(new Character(c));
            } else {
                charIndices.add(i);
                characters.add(new Character(c));
            }
        }
        
        // N
        Collections.reverse(digits);
        Character[] resultArray = new Character[input.length()];
        
        // M
        for(int i=0; i < digitsIndices.size(); i++){
            int index = digitsIndices.get(i);
            Character value = digits.get(i);
            
            resultArray[index] = value;
        }
        
        // P
        for(int i=0; i < charIndices.size(); i++){
            int index = charIndices.get(i);
            Character value = characters.get(i);
            
            resultArray[index] = value;
        }
        
        // M + P = N
        
        StringBuilder output = new StringBuilder();
        
        // N
        for (int i=0; i < resultArray.length; i++){
            output.append(resultArray[i]);
        }
        
        return output.toString();
    }
    
    
    public static void main(String[] args) {
        System.out.println(reverseDigits("abcdefg"));
        // System.out.println(reverseDigits("ab4c13"));
    }
}