287. Find the Duplicate Number

There is only one repeated number in nums, return this repeated number.

You must solve the problem without modifying the array nums and uses only constant extra space.

O(N^2) Brute force

• check for all numbers if duplicate number exists.

O(NlogN) Sorting

• Sort the array and duplicate numbers will be next to each other.

O(N) Time O(N) space, Hash Table

• With the help of hash (set/map/vector) we can find the duplicate number.

Floyd’s Cycle Detection Algorithm

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);

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