花花酱 LeetCode 2216. Minimum Deletions to Make Array Beautiful

You are given a 0-indexed integer array nums. The array nums is beautiful if:

• nums.length is even.
• nums[i] != nums[i + 1] for all i % 2 == 0.

Note that an empty array is considered beautiful.

You can delete any number of elements from nums. When you delete an element, all the elements to the right of the deleted element will be shifted one unit to the left to fill the gap created and all the elements to the left of the deleted element will remain unchanged.

Return the minimum number of elements to delete from nums to make it beautiful.

Example 1:

Input: nums = [1,1,2,3,5]
Output: 1
Explanation: You can delete either nums[0] or nums[1] to make nums = [1,2,3,5] which is beautiful. It can be proven you need at least 1 deletion to make nums beautiful.

Example 2:

Input: nums = [1,1,2,2,3,3]
Output: 2
Explanation: You can delete nums[0] and nums[5] to make nums = [1,2,2,3] which is beautiful. It can be proven you need at least 2 deletions to make nums beautiful.

Constraints:

• 1 <= nums.length <= 105
• 0 <= nums[i] <= 105

Solution: Greedy + Two Pointers

If two consecutive numbers are the same, we must remove one. We don’t need to actually remove elements from array, just need to track how many elements have been removed so far.

i is the position in the original array, ans is the number of elements been removed. i – ans is the position in the updated array.

ans += nums[i – ans] == nums[i – ans + 1]

Remove the last element (just increase answer by 1) if the length of the new array is odd.

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