gistfile1.java

class Solution{
    /*
    * Space complexity: O(n); Time complexity: O(n).
    */
	  
	  public static String compression(String str){
	    if(str == null || str.isEmpty()) return str;
	    
	    char[] chars = str.toCharArray();
	    StringBuffer result = new StringBuffer();
	    int count = 1; // Count the first char.
	    
	    for(int i = 0; i<chars.length - 1; i++){
	      if(chars[i] == chars[i+1]) count++;
	      
	      if(chars[i] != chars[i+1] || i==chars.length - 2){ // When the next char is different or the end, append the char and its count to result.
	        result.append(chars[i]);
	        result.append(count);
	        count = 1; // Start to count the next different char.
	      }
	    }
	    
	    return result.length() < str.length()? result.toString() : str;
	    
	  }
	  
}