【leetcode】 Longest Substring Without Repeating Characters

int lengthOfLongestSubstring(string s) {
	int len = s.size();
	if(len <= 1)
		return len;
	//hash_table,record the index of items in s
	int hash_table[255];
	//-1 means haven't occur  
	memset(hash_table, -1, sizeof(hash_table));

	int start = 0;
	int end = 0;
	int max_len = 0;
	while(end < len)
	{
		char c = s[end];
		if(hash_table[c] == -1)
				hash_table[c] = end++;
		else
		{
			max_len = max(max_len, end - start);
			//new start
			start = hash_table[c] + 1;
			//clean items in hash_table .
			memset(hash_table, -1, sizeof(hash_table));
			//record the index of items from new start to end.
			for(int i = start; i <= end; ++i)
				hash_table[s[i]] = i;
			++end;
		}
	}
	//may be the s without repeating character from start to end.
	max_len = max(max_len, end - start);
	return max_len;
}