3. Longest Substring Without Repeating Characters LeetCode

 Time Complexity: O(N) 

class Solution {
	public:
	int lengthOfLongestSubstring(string s) {
	vector< int >cnt(256,0);
	int mx = 0;
	int st = 0;
	for(int i=0; i<(int)s.size(); i++){
	cnt[s[i]]++;
	if(cnt[ s[i] ] > 1){
	for(int j=st; j<i; j++){
	cnt[ s[j] ]--;
	if( s[i] == s[j] ){
	st = j+1;
	break;
	}
	}
	}
	mx = max(mx, i - st + 1);
	}
	return mx;
	}
	};