//双指针--快慢指针
class Solution {
public:
    int duplicateInArray(vector<int>& nums) {
        int slow=0,fast=0;
        while(true){
            slow=nums[slow];
            fast=nums[nums[fast]];
            if(slow==fast){
                slow=0;
                while(slow!=fast) slow=nums[slow],fast=nums[fast];
                return slow;
            }
        }
        return -1;
    }
};