class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        string res = "";
        int maxLen = 0;
        // 1、双指针i，j
        int i = 0;
        int j = 0;
        while (j < s.length()) {
            while (j < s.length() && (res.find(s[j]) == string::npos )) {
                res += s[j];
                j ++;
            }
            maxLen = max(j - i, maxLen);
            res = "";
            i ++;
            j = i;
        }
        return maxLen;

    }
};