25.findDuplicateNumber.cpp

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