/**
 * @param {string} s
 * @return {number}
 */
var lengthOfLongestSubstring = function(s) {
  let n = s.length;
  
  let charSet = new Set();
  
  let longest = 0;
  let i = 0;
  let j = 0;
  
  while (i < n && j < n){
    if (!charSet.has(s.charAt(j))){
      charSet.add(s.charAt(j));
      ++j;
      if (j - i > longest){
        longest = j - i;
      }
    } else {
      charSet.delete(s.charAt(i));
      ++i;
    }
  }
  
  return longest;
};