1911 - Maximum Alternating Subsequence Sum.

Medium

The alternating sum of a 0-indexed array is defined as the sum of the elements at even indices minus the sum of the elements at odd indices.

• For example, the alternating sum of [4,2,5,3] is (4 + 5) - (2 + 3) = 4.

Given an array nums, return the maximum alternating sum of any subsequence of nums (after reindexing the elements of the subsequence).

A subsequence of an array is a new array generated from the original array by deleting some elements (possibly none) without changing the remaining elements' relative order. For example, [2,7,4] is a subsequence of [4,2,3,7,2,1,4] (the underlined elements), while [2,4,2] is not.

Example 1:

Input: nums = 4,2,5,3

Output: 7

Explanation: It is optimal to choose the subsequence 4,2,5 with alternating sum (4 + 5) - 2 = 7.

Example 2:

Input: nums = 5,6,7,8

Output: 8

Explanation: It is optimal to choose the subsequence 8 with alternating sum 8.

Example 3:

Input: nums = 6,2,1,2,4,5

Output: 10

Explanation: It is optimal to choose the subsequence 6,1,5 with alternating sum (6 + 5) - 1 = 10.

Constraints:

• <code>1 <= nums.length <= 10⁵</code>

• <code>1 <= numsi<= 10⁵</code>