题目
参考《算法小抄》重的解法，重点理解！！！

class Solution {
    public int lengthOfLongestSubstring(String s) {
        if (s.length() < 2) {
            return s.length();
        }
        char[] array = s.toCharArray();
        int left = 0, right = 0, res = 0;
        int[] count = new int[256];
        while (right < array.length) {
            char cur = array[right];
            ++right;
            ++count[cur];
            while (count[cur] > 1) {
                char d = array[left];
                ++left;
                --count[d];
            }

            res = Math.max(res, right - left);
        }

        return res;
    }
}