【题意】

Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

【思路】

一开始我是用 HashMap 做的，然后就在考虑 字母做key还是下标做key，总之比较麻烦。

后来看到网上普遍用数组来实现 HashMap 的功能，感觉还是挺新的。至于为什么数组的大小设为256，始终想不明白。我查了下ASCII码表，上面也只有127个字符。A~Z是65~90，a~z是97~112。

下面的代码我觉得是已经相当简洁了，参考出处：Java，C++，大家共同学习。

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        int len = s.length();
        if (s == null || len == 0) return 0;
        
        int[] table = new int[256];
        Arrays.fill(table, -1);
        
        int maxlen = 1;
        int begin = 0, end = 1;
        table[s.charAt(0)] = 0; //important!
        while (end < len) {
            char endch = s.charAt(end);
            if (table[endch] >= begin) {
                begin = table[endch] + 1;
            }
            table[endch] = end;
            maxlen = Math.max(maxlen, end-begin+1);
            end++;
        }
        
        return maxlen;
    }
}

【LeetCode】Longest Substring Without Repeating Characters 解题报告