Java compressing Strings

Question

I need to create a method that receives a String and also returns a String.

Ex input: AAABBBBCC

Ex output: 3A4B2C

Well, this is quite embarrassing a

Answer 1

The easiest approach:- Time Complexity - O(n)

public static void main(String[] args) {
    String str = "AAABBBBCC";       //input String
    int length = str.length();      //length of a String

    //Created an object of a StringBuilder class        
    StringBuilder sb = new StringBuilder(); 

    int count=1;   //counter for counting number of occurances

    for(int i=0; i<length; i++){
        //if i reaches at the end then append all and break the loop
        if(i==length-1){         
            sb.append(str.charAt(i)+""+count);
            break;
        }

        //if two successive chars are equal then increase the counter
        if(str.charAt(i)==str.charAt(i+1)){   
            count++;
        }
        else{
        //else append character with its count                            
            sb.append(str.charAt(i)+""+count);
            count=1;     //reseting the counter to 1
        }
   }

    //String representation of a StringBuilder object
    System.out.println(sb.toString());   

}

Answer 2

O(n) approach

No need for hashing. The idea is to find the first Non-matching character. The count of each character would be the difference in the indices of both characters.

for a detailed answer: https://stackoverflow.com/a/55898810/7972621

The only catch is that we need to add a dummy letter so that the comparison for the last character is possible.

private static String compress(String s){
    StringBuilder result = new StringBuilder();
    int j = 0;
    s = s + '#';
    for(int i=1; i < s.length(); i++){
        if(s.charAt(i) != s.charAt(j)){
            result.append(i-j);
            result.append(s.charAt(j));
            j = i;
        }
    }
   return result.toString();
}