0287.寻找重复数

https://leetcode.cn/problems/find-the-duplicate-number/

方法一：快慢指针

时间复杂度 $O(n)$，空间复杂度 $O(1)$。

impl Solution {
    pub fn find_duplicate(nums: Vec<i32>) -> i32 {
        let mut slow = nums[0] as usize;
        let mut fast = nums[nums[0] as usize] as usize;
        while slow != fast {
            slow = nums[slow] as usize;
            fast = nums[nums[fast] as usize] as usize;
        }
        let mut slow = 0 as usize;
        let mut fast = fast as usize;
        while slow != fast {
            slow = nums[slow] as usize;
            fast = nums[fast] as usize;
        }
        slow as i32
    }
}