寻找重复数

/**
 * @param {number[]} nums
 * @return {number}
 */
var findDuplicate = function(nums) {
  let slow = 0
  let fast = 0
  do {
    slow = nums[slow]
    fast = nums[nums[fast]]
  } while (slow !== fast)
  slow = 0
  while (slow !== fast) {
    slow = nums[slow]
    fast = nums[fast]
  }
  return slow
};