function isValid(s: string): string {
  const charFreq = new Map();
  
  for (let char of s) {
    charFreq.set(char, (charFreq.get(char) || 0) + 1);
  }
  
  const freqCount = new Map();
  for (let freq of charFreq.values()) {
    freqCount.set(freq, (freqCount.get(freq) || 0) + 1);
  }
  
  if (freqCount.size === 1) return "YES";
  if (freqCount.size > 2) return "NO";
  
  const [[freq1, count1], [freq2, count2]] = Array.from(freqCount.entries());
  
  // Check valid conditions
  return (
    (freq1 === 1 && count1 === 1) ||
    (freq2 === 1 && count2 === 1) ||
    (Math.abs(freq1 - freq2) === 1 && (count1 === 1 || count2 === 1))
  ) ? "YES" : "NO";
}