LeetCode(3):Longest Substring Without Repeating Characters

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.

题意：求最长非重复字串。

思路：采用双指针进行解决，利用一个数组保存字符是否出现，从前向后遍历数组，如果遇到已存在的字符，应该回退到这个字符出现的下一个位置重新开始统计，同时要更新数组。

代码：

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        if(s==null||s.length()==0) return 0;
        int res = 1;
        boolean[] exist = new boolean[256];
        int start = 0;
        for(int i =0;i<s.length();i++){
            char ch = s.charAt(i);
            if(exist[ch]){
                res = Math.max(res,i-start);
                for(int k=start;k<i;k++){
                    if(s.charAt(k)==ch){
                        start = k + 1;
                        break;
                    }
                    exist[s.charAt(k)] = false;
                }
            }else{
                exist[ch] = true;
            }
        }
        res = Math.max(res,s.length() - start);
        return res;
    }
}