Number of Unequal Triplets in Array

class Counter {
  private readonly map: Map<number, number>;
 
  constructor() {
    this.map = new Map();
  }
 
  get(num: number) {
    return this.map.get(num) ?? 0;
  }
 
  add(num: number) {
    return this.map.set(num, this.get(num) + 1);
  }
 
  sub(num: number) {
    return this.map.set(num, this.get(num) - 1);
  }
}
 
export function unequalTriplets(nums: number[]): number {
  const counter = new Counter();
 
  let pairs = 0;
  let triplets = 0;
  nums.forEach((num, i) => {
    const count = counter.get(num);
    triplets += pairs - count * (i - count);
    pairs += i - count;
    counter.add(num);
  });
 
  return triplets;
}