#include <string>
#include <unordered_set>
#include <iostream>
#include <algorithm>
 
using namespace std;
 
int lengthOfLongestSubstring(string s) {
    unordered_set<char> sub;
    size_t max_len = 0;
 
    std::for_each(s.begin(), s.end(), [&](char ch)
    {
        if (sub.count(ch))
        {
            max_len = std::max(sub.size(), max_len);
            sub.clear();
        }
        sub.insert(ch);
    });
    return max_len;
}
 
int main()
{
    std::cout << lengthOfLongestSubstring("nnnnnn");
}

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