3. Longest Substring Without Repeating Characters

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.

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        int max = 0, i =0 , j = 0,index;
        int[] hm = new int[128];    //ASCII字符表128个，用来表示某字符当前位置
        while(j<s.length()){     //进行一一遍历
            if(hm[s.charAt(j)] == 0){     //如果没有出现过
                hm[s.charAt(j)] = j+1;        //标记为当前位置
                j++;
                if((j-i) > max)     //j-i为此次寻找的字符串的长度，取最大值
                    max = j-i;
            }
            else {                           //如果出现过
                index = hm[s.charAt(j)];    //index为当前位置
                hm[s.charAt(j)] = 0;
                if (i < index)
                    i = index;
            }
        }
        return max;
    }
}