思路：
滑動窗口

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int ret=0,left=0,right=-1;
        int rec[256]={0};
        int n=s.size();
        while(right+1<n){
             if(rec[s[right+1]]==0){
                rec[s[++right]]=1;
                 if(ret<right-left+1)
                     ret=right-left+1;
             }
            else{
                rec[s[left++]]=0;
            }
        }
        return ret;
    }
};