public class Reversing {

 public static void main(String[] args) {
  String str = "Nitin";
  
  // If str is quite small then StringBuilder will be quite slow as compare
  // to custom approach given below. For large values StringBuilder is quite
  // fast than the custom approach below.
  long start = System.nanoTime();
  StringBuilder sb1 = new StringBuilder(str);
  sb1.reverse();
  System.out.println("Time Taken : " + (System.nanoTime() - start));
  
  start = System.nanoTime();
  int len = str.length();
  StringBuilder sb = new StringBuilder(len);

  int j = 0;
  for(int i = (len/2-1); i >= 0; i--) {

    sb.append(str.charAt(i));
    sb.insert(j, str.charAt(len-j-1));
    j++;
  }
  sb.insert((len/2), str.charAt((len/2)));
  System.out.println("Time Taken : " + (System.nanoTime() - start));
  
  System.out.println(sb);
 }
}

Or better use below way to save memory & have better performance -