3. Longest Substring Without Repeating Characters

采用双指针法：首尾指针移动，并保证首尾之间无重复元素出现；当有重复元素出现时，判断maxLen是否需要更新；

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        if(s.size()<1)
            return 0;
        
        int begin = 0, maxLen=1,end=begin;
        unordered_map<char,bool>  table;
        while(end<s.size())
        {
            while(table.find(s[end])==table.end()&&end<s.size())
            {
                table[s[end]]==true;
                end++;              
            }
           
            table.erase(table.begin(),table.end());
            maxLen=max(maxLen,end-begin);
            begin++;
            end = begin;                      
        }
        
        if(end == s.size())
            maxLen = max(maxLen,end-begin);
        
        return maxLen;
    }
};