287-Find-the-Duplicate-Number.js

function findDuplicate(nums) {
  let slow = 0;
  let fast = 0;

  while (true) {
    slow = nums[slow];
    fast = nums[nums[fast]];
    if (slow == fast) {
      break;
    }
  }

  let slow2 = 0;

  while (true) {
    slow = nums[slow];
    slow2 = nums[slow2];
    if (slow == slow2) {
      break;
    }
  }

  return slow;
}