最长不含重复字符的子字符串（Java实现）

public class E48LongestSubstringWithoutDuplication {
    //最长不含重复字符的子字符串
    public static int count(String str){
        if (str == null || str.length() == 0)
            return -1;
        //记录返回值
        int maxLength = 0;
        //记录当前长度
        int currentLength = 0;
        //记录当前字符上一次出现的位置
        int[] positions = new int[26];
        for (int i = 0; i < 26; i ++)
            positions[i] = -1;

        for (int i = 0; i < str.length(); i ++){
            int preposition = positions[str.charAt(i) - 'a'];
            if (preposition < 0 || i - preposition > currentLength){
                currentLength ++;
            }
            else{
                if (currentLength > maxLength)
                    maxLength = currentLength;
                currentLength = i - preposition;
            }
            positions[str.charAt(i) - 'a'] = i;
        }
        if (currentLength > maxLength)
            maxLength = currentLength;
        return maxLength;
    }

    //测试用例
    public static void main(String[] args){
        String str = "arabcacfr";
        System.out.println(E48LongestSubstringWithoutDuplication.count(str)); //4
    }
}