3. Longest Substring Without Repeating Characters

int lengthOfLongestSubstring(string s) {

	int n = s.size();
	if (n <= 1)
	{
		return n;
	}
	int i = 0;//start of the current checking string
	int j = 1;//end of the current checking string
	int len = 0;
	int max_len = 0;
	while (j < n){
		for (int k = i; k < j; ++k){//k is current index between i and j
			if (s[j] == s[k]){ //always comparing from i...k...j
				max_len = max_len>len ? max_len : len;//if find a duplicated one check which is the larger one
				len = 1; //start to re-calculate length
				++i;	//start next checking from j
						//i = j won't work at cornercase:"abac"
				j = i + 1;	//j goto to next element
				break;
			}
			else{
				if (k == j -1){ //make sure it's the last char in this turn 
					len = j - i + 1;//calculate length only when  compared the last char in this turn e.g. "abb"
					++j;
					break; // here needs a break. if not, j+1, for loop will continue, but actually we finished the check in this turn 
				}
			}
		}
	}

	//if program goes here, that means we checked the whole string(j is at the end of string s).
	//maybe there are duplicated words, maybe not. So we need to check len and max_len
	len = j - i; // here, len shouldn't be j - i + 1, because before while loop is over, j already +1
	return max_len>len ? max_len : len;
}

只想到了这个方法，但因为大字符串，计算超时，没有通过leetcode。