Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

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.

Input: s = ""
Output: 0

int lengthOfLongestSubstring(string s){
    vector<int> dict(256,-1);
    int maxLen = 0, start = -1;
    for (int i=0; i!=s.length(); i++){
        if (dict[s[i]] > start)
            start = dict[s[i]];
        dict[s[i]] = i;
        maxLen = max(maxLen, i-start);
    }
    return maxLen;
}

   int lengthOfLongestSubstring(string s) {
        int n=s.length();
        if(n==0)
            return 0;
        set<char> st;
        int maxsize=0;
        int i=0,j=0;
        while(j<n)
        {
            if(st.count(s[j])==0)
            {
                st.insert(s[j]);
                maxsize=max(maxsize,(int)st.size());
                j++;
            }
            else
            {
                st.erase(s[i]);
                i++;
            }
        }
        return maxsize;
    }