#384 Longest Substring Without Repeating Characters

题目描述：

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

Have you met this question in a real interview? 

Yes

Example

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.

Challenge 

O(n) time

题目思路：

这题还是沿用two pointers的思路：用两个pointers l和r，初始时一直在string的头上。然后r开始走，并且把r走到的char都加入到一个map中：如果map中已经有了这个char，则通过l++来缩短substring，并且把l走到的char都从map中删去，直到map删去了这个重复的char。这就是没有重复char的substring之一，计算它的长度，并且和max_len取max。然后，r就又可以走下去，进行下一个循环了。

Mycode（AC = 17ms）：

class Solution {
public:
    /**
     * @param s: a string
     * @return: an integer 
     */
    int lengthOfLongestSubstring(string s) {
        // write your code here
        int helper[256] = {0,};
        int l = 0, r = 0, max_len = 0;
        
        while (r < s.length()) {
            // add char into the map
            helper[int(s[r])] += 1;
            
            // delete char from start of substring
            // if the s[r] is already in the map
            while (helper[int(s[r])] > 1) {
                helper[int(s[l++])] -= 1;
            }
            
            // increase r to find next substring
            r++;
            
            // get max length
            max_len = max(max_len, r - l);
        }
        
        return max_len;
    }
};