题目：

题解：

func findDuplicate(nums []int) int {
    slow, fast := 0, 0
    for slow, fast = nums[slow], nums[nums[fast]]; slow != fast; slow, fast = nums[slow], nums[nums[fast]] { }
    slow = 0
    for slow != fast {
        slow = nums[slow]
        fast = nums[fast]
    }
    return slow
}