最长不含重复字符的子字符串

题目：

请从字符串中找出一个最长的不含重复字符串的子字符串，计算该最长子字符串的长度。假设字符串中只包含'a'-'z'的字符。

解答：

public class Solution {

    public static void main(String[] args) {
        String str "arabcacfr";
        System.out.println(findLongestSubstringLength(str));
    }

    public static int findLongestSubstringLength(String str) {
        if(str == null || str.equals("")) {
            return 0;
        }

        int maxLength = 0;
        int curLength = 0;

        int[] positions = new int[256];
        for(int i = 0; i < positions.length; i++) {
            positions[i] = -1;
        }

        for(int i = 0; i < str.length(); i++) {
            int curChar = str.charAt(i) - 'a';

            int prePosition = positions[curChar];
            int distance = i - prePosition;

            // 第一次出现或前一个非重复子字符串不包含当前字符
            if(prePosition < 0 || distance > curLength) {
                curLength++;
            } else {
                if(curLength > maxLength) {
                    maxLength = curLength;
                }

                curLength = distance;
            }

            positions[curChar] = i;
        }

        if(curLength > maxLength) {
            maxLength = curLength;
        }

        return maxLength;
    }
}