Find the Duplicate Number

export function findDuplicate(nums: number[]): number {
  let slow = nums[0];
  let fast = nums[nums[0]];
 
  while (slow !== fast) {
    slow = nums[slow];
    fast = nums[nums[fast]];
  }
 
  slow = 0;
  while (slow !== fast) {
    slow = nums[slow];
    fast = nums[fast];
  }
 
  return slow;
}