287_findDuplicateNumber.cpp

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