Remove Duplicates from Sorted Array II

The key idea

Because the array is sorted, every group of equal values is contiguous. A value at read index fast may be kept only if it differs from the value two slots back in the already-written prefix, i.e. nums[fast] != nums[slow-2]. That single test caps each value at two copies without counting anything.

Problem

Given an integer array nums sorted in non-decreasing order, remove some duplicates in-place so that each unique element appears at most twice. The relative order of the elements should be kept the same.

Since it is impossible to change the length of the array in some languages, you must instead have the result be placed in the first part of the array nums. More formally, if there are k elements after removing the duplicates, then the first k elements of nums should hold the final result. It does not matter what you leave beyond the first k elements.

Return k after placing the final result in the first k slots of nums.

You must do this by modifying the input array in-place with O(1) extra memory.

Constraints

1 <= nums.length <= 3 * 10^4
-10^4 <= nums[i] <= 10^4
nums is sorted in non-decreasing order.

Examples

Input: nums = [1,1,1,2,2,3] Output: 5, nums = [1,1,2,2,3,_]

Input: nums = [0,0,1,1,1,1,2,3,3] Output: 7, nums = [0,0,1,1,2,3,3,_,_]

Complexity

Time: O(n) Space: O(1)

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