QUICK SELECT ALGORITHM

QuickSelect.js

const quickSelect = (nums, k, kthMax) => {
  if (!nums.length) return;

  const P = nums[Math.floor(Math.random() * nums.length)];

  const left = [];
  const mid = [];
  const right = [];

  nums.forEach(x => {
    if (x === P) mid.push(x);
    else if (kthMax ? x > P : x < P) left.push(x);
    else right.push(x);
  });

  const L = left.length;
  const M = mid.length;

  if (k <= L) return quickSelect(left, k, kthMax);
  else if (k > L + M) return quickSelect(right, k - L - M, kthMax);
  return mid[0];
}

console.log(quickSelect([3, 2, 1, 5, 6, 4], 2, false)); // Output: 2 (2nd smallest)
console.log(quickSelect([3, 2, 3, 1, 2, 4, 5, 5, 6], 4, true)); // Output: 4 (4th largest)