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class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        int slow, fast;
        slow = nums[0]; fast = nums[0];

        do{
            slow = nums[slow];
            fast = nums[nums[fast]];
        }while(slow != fast);

        slow = nums[0];
        while(slow != fast){
            slow = nums[slow];
            fast = nums[fast];
        }
        return slow;
    }
};