【剑指offer】字符流中第一个不重复的字符

private int[] occurrence = new int[256];
private int index = 0;

public void (char ch) {
    index++;
    int current = (int) ch;

    if (occurrence[current] == 0)
        occurrence[current] = index;
    else if (occurrence[current] > 0)
        occurrence[current] = -1;
}

public char FirstAppearingOnce() {
    char result = '