leetcode-com-explore-challenge-card-june-leetcoding-challenge-542-week-4-june-22nd-june-28th-3371.cpp

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        int slow=nums[0];
        int fast=nums[0];
        
       do{
           slow=nums[slow];
           fast=nums[nums[fast]];
       }
        while(slow!=fast);
        
        slow=nums[0];
        while(slow!=fast){
            fast=nums[fast];
            slow=nums[slow];
        }
        return fast;
    }
};