LeetCode 287. Find the Duplicate Number

class Solution {
public:
    int findDuplicate(vector<int>& nums) {

        // Find the intersection point of the two runners.
        int tortoise = nums[0];
        int hare = nums[0];

        do {
            tortoise = nums[tortoise];
            hare = nums[nums[hare]];
        } while (tortoise != hare);

        // Find the "entrance" to the cycle.
        tortoise = nums[0];
        while (tortoise != hare) {
            tortoise = nums[tortoise];
            hare = nums[hare];
        }

        return hare;
    }
};