LengthOfLongestSubstring

var lengthOfLongestSubstring = function(s) {
  let start = 0, maxLen = 0;
  const map = new Map();

  for(let i = 0; i < s.length; i++) {
      const ch = s[i];
    
      if(map.get(ch) >= start) start = map.get(ch) + 1;
      map.set(ch, i);
    
      if(i - start + 1 > maxLen) maxLen = i - start + 1;
  }

  return maxLen;
};