209. first unique character in a string

public class  {
    
     * @param str: str: the given string
     * @return: char: the first unique character in a given string
     */
    public char firstUniqChar(String str) {
        // Write your code here
        int[] freq = new int[256];
        for (int i = 0; i < str.length(); i++) {
            freq[str.charAt(i)]++;
        }
        for (int i = 0; i < str.length(); i++) {
            if (freq[str.charAt(i)] == 1) {
                return str.charAt(i);
            }
        }
        return Character.MIN_VALUE;

    }
}