Longest Substring Without Repeating Characters

一、问题描述

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.
二、思路

开始没有用辅助空间，导致不能正常AC，后来果断采用辅助空间。

原理是：采用两个标量i和start，i代表一个快指针，start代表慢指针，子串的长度即为i - start，当第二次出现某个字符时，start才会移动。

三、代码

class Solution {
public:
int lengthOfLongestSubstring(string s){
vector<int> v(256,-1);
int max_len = 0; int start = -1;
for(int i = 0; i < s.size(); ++i){
if(v[s[i]] > start)
start = v[s[i]];
v[s[i]] = i;
max_len = max(max_len, i - start);
}
return max_len;
}
};