letcode 3：the longest substring

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        //先得到字符串长度
        int s_len = s.length() - 1;
        int counti = 1; //从第二个开始查找
        int countj;
        int count_diff = 1; //计数不相同的字母
        int max_len = 1;    //最长长度一开始为1
        int down = 0;   //一开始从最底下开始查询某个字母的是否在他前面有相同，遇见相同的就截断，重新赋值down
        if ( s_len < 0)
            max_len = 0;
        else{
            while ( counti <= s_len ){  //一遍将s查询完
                for ( countj = down; countj < counti; countj++){
                    if ( s[countj] != s[counti] ){
                        count_diff++;
                        if (max_len < count_diff){
                            max_len = count_diff;
                        }
                    }
                     else {
                         down = countj + 1; //从重复的字母后面哪一个开始查
                     }       
                }
                counti++;
                count_diff = 1; //重置计数器，以便于后面计算
            }
        }
        return max_len;         
    }
};