Programming Questions on String and its Best Solution with its complexity

Question: Write an algorithm to find if a string has all unique characters.what if you cannot use additional data-structure?.

Solution: i will write only method(Algorithm).

public boolean isUniqueChar(String str)

{

   if(str.lenght() > 256)

    return false;

 boolean[] char_set = new boolean[256];

 for(int a=0; str.length(); a++)

  {

    int val = str.charAt(a);

    if(char_set[val])

    {

       return false;   // because alredy found this char instring

    }

     char_set[val] = true;

  }

  return true;

}

Explanation:

 Time Complexity for this code is o(n) where  n=length of the string.

  Space complexity =o(1)

Again once question here,Can we reduce the space complexity? if yes then how?

Solution: yes we can reduce the space complexity using a bit vector.