Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be preserved.

To meet the requirements of an in-place solution:

Modify the input array nums such that the first k elements contain the unique elements, in the same order as they first appeared.

The elements beyond index k - 1 do not matter and can be ignored. The length of nums does not change; only the first k positions are evaluated.

After removing the duplicates, return k, the number of unique elements.

Example 1:

Input: nums = [1, 1, 3]
Output: 2
Explanation: 
Your function should return k = 2, with the first two elements of nums being 1 and 3, respectively. 
Actual output array: [1, 3, 3].
One valid conceptual output array: [1, 3, _].
The value beyond index k - 1 does not matter and can be ignored, represented here with an underscore.

Example 2:

Input: nums = [0, 0, 1, 1, 2, 2, 2, 3, 4, 4]
Output: 5
Explanation:    
Actual output array: [0, 1, 2, 3, 4, 2, 2, 3, 4, 4]
Conceptual output array: [0, 1, 2, 3, 4, _, _, _, _, _]
Your function should return k = 5, with the first five elements of nums being 0, 1, 2, 3, 4 in that order.

Constraints

1 <= nums.length <= 3 × 10⁴

-100 <= nums[i] <= 100
nums is sorted in non-decreasing order.

21. Number of Unique Elements in Array