3. Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters.

Examples:

Given "abcabcbb", the answer is "abc", which the length is 3.

Given "bbbbb", the answer is "b", with the length of 1.

Given "pwwkew", the answer is "wke", with the length of 3. Note that the answer must be a substring, "pwke" is a subsequence and not a substring.

public class Solution {    public int lengthOfLongestSubstring(String s) {         int[] last = new int[128];         int start = 0;        int len = 0;        char[] w = new char[s.length()];        w = s.toCharArray();        for(int i = 0; i < 128; i++)            last[i] = -1;//last数组用于保存新出现的字符的下标，一开始全部初始化为-1        for(int i = 0; i < s.length(); ++i){            if(last[w[i]-' '] >= start){ //当前这个字符出现过                if(i-start > len)                    len = i-start;                start = last[w[i]-' '] + 1; //从这个字符首次出现的位置+1，重新扫描，相当于把前面抛开前面的字符串不谈            }                last[w[i]-' '] = i;//更新当前字符的下标        }        if(len > s.length() - start)//针对没有重复字符的字符串            return len;        else            return s.length() - start;    }}