Compress a string

Write a program to compress a string. If input is "aabbb" expected output is "a2b3"

The approach:-

1. Have 2 pointers read and write to the input string.
2. Scan the string using read pointer.
3. If similar characters are seen keep incrementing a count.
4. If character sequence changes, write the character and the count value at the write pointer.
5. Finally terminate the string with a null character.

C++ program to compress a string

#include <iostream>
using namespace std;

int main() 
{ 
	char input[] = "aaabbcccddeeee"; 
	char* read = input; 
	char* write = input; 

	char c = input[0]; 
	int count = 0; 
	while ( *read != '\0' ) {  
		if ( *read == c ) {   
			count++;  
		}  
		else {   
			*write++ = c;   
			*write++ = count + '0';   
			count = 1;  
		}  
		c = *read;  
		read++; 
	} 
	*write++ = c; 
	*write++ = count + '0'; 
	*write = '\0'; 
	cout << input << endl; 
	return 0;
}

Output:-

a3b2c3d2e4

Java program to compress a string

In Java, an additional StringBuilder could be used to store the output since String is immutable. Otherwise the approach to compress is the same.

package com.stackstalk;

public class CompressString {

  public static String compress(String input) {
	  
    StringBuilder output = new StringBuilder();
		
    if ( input.length() == 0 )
        return output.toString();
		
    char c = input.charAt(0);		
    int count = 0;
		
    for ( int index = 0; index < input.length(); index++ ) {
        if ( input.charAt(index) == c ) 
            count++;			
    else {
            output.append(c);
            output.append(count);
            c = input.charAt(index);
            count = 1;
        }
    }
		
    output.append(c);
    output.append(count);
		
    return output.toString();
  }
	
  public static void main(String[] args) {
    System.out.println(compress("a"));
    System.out.println(compress("aaddbbdd"));
    System.out.println(compress("aabbbbccddde"));
  }
}

Output:

a1
a2d2b2d2
a2b4c2d3e1