Length of the longest substring without repeating characters

Given a string, find the length of the longest substring without repeating characters. For example, the longest substrings without repeating characters for “ABDEFGABEF” are “BDEFGA” and “DEFGAB”, with length 6.

public class LongestSubString {

 public static void main(String[] args) {
  String str = "ABDEFGABEF";

  // Used for tracking the position of last occurrence of character in the
  // string.
  int[] chars = new int[256];
  for (int i = 0; i < 256; i++) {
   chars[i] = -1;
  }

  int maxLength = 1;
  int currentLength = 1;

  for (int i = 0; i < str.length(); i++) {
   int lastOccrrenceIndex = chars[str.charAt(i)];
   if (lastOccrrenceIndex == -1
     || (i - currentLength > lastOccrrenceIndex)) {
    currentLength++;
   } else {
    if (currentLength > maxLength)
     maxLength = currentLength;
    currentLength = i - lastOccrrenceIndex;
   }
   chars[str.charAt(i)] = i;
  }
  if (currentLength > maxLength)
   maxLength = currentLength;
  System.out.println(maxLength);
 }
}

Time Complexity: O(n + d) where n is length of the input string and d is number of characters in input string alphabet. For example, if string consists of lowercase English characters then value of d is 26.
Auxiliary Space: O(d)

Refer : http://www.geeksforgeeks.org/archives/15859