Find the longest unique-character window

Grow a fresh set from every possible start. This is straightforward but repeats work.

Time: Expected O(n²) · Space: O(n)

function solve(input) {
 const chars=Array.from(input);let best=0;
 for(let left=0;left<chars.length;left++){
  const seen=new Set();
  for(let right=left;right<chars.length;right++){
   if(seen.has(chars[right]))break;
   seen.add(chars[right]);best=Math.max(best,right-left+1);
  }
 }
 return best;
}

Track the last index at which each character appeared. When a duplicate falls inside the active window, advance the left boundary past its previous occurrence. The boundary must never move backwards. Array.from iterates Unicode code points, so this exercise treats an emoji code point as one item; user-perceived grapheme clusters require a different segmentation policy.

Time: Expected O(n) · Space: O(n), including the code-point array.

function solve(input) {
  const chars = Array.from(input), last = new Map();
  let left = 0, best = 0;
  for (let right=0; right<chars.length; right++) {
    const previous = last.get(chars[right]);
    if (previous !== undefined) left = Math.max(left, previous + 1);
    last.set(chars[right], right);
    best = Math.max(best, right - left + 1);
  }
  return best;
}