longest substring without repeating characters

#include <vector>
using namespace std;

int (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 main()
{
	cout<<length("abcabcbb")<<endl;
}

Output: 3