3. Longest Substring Without Repeating Characters最长子串

难度：Medium
问题描述：Given a string s, find the length of the longest substring without repeating characters.

Example 1:
input: s = “abcabcbb”
Output: 3
Explanation: The answer is “abc”, with the length of 3.
Example 2:
Input: s = “bbbbb”
Output: 1
Explanation: The answer is “b”, with the length of 1.
Example 3:
Input: s = “pwwkew”
Output: 3
Explanation: The answer is “wke”, with the length of 3.
Notice that the answer must be a substring, “pwke” is a subsequence and not a substring.
Example 4:
Input: s = “”
Output: 0

思路：
重点在于理解最长子串中子串必须是连续的，连续问题可用滑动窗口解决，窗口中的串就是所求，注意若出现重复的字符，应将该字符串从该窗口中删除，返回窗口中子串的长度时，需要一个标量left记住窗口最左端的位置，则窗口的最右端便是当前访问的位置。

解法1：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int re=0;
        int left = -1;
        int size = s.size();
        unordered_map<int,int> m;
        for(int i=0;i<size;i++)
        {
            if(m.count(s[i]) && m[s[i]]>left)
            {
                left = m[s[i]];
            }
            m[s[i]] = i;
            re = max(re,i-left);
        }
        return re;
    }
};

解法2：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int re = 0;
        int left = -1;
        int size = s.size();
        vector<int> vec(128,-1);
        for(int i=0;i<size;i++)
        {
            left = max(left,vec[s[i]]);
            vec[s[i]] = i;
            re = max(re,i-left);
        }
        return re;
    }
};