feat: add solutions to lc problems: No.2815~2818 (doocs#1440)

* No.2815.Max Pair Sum in an Array
* No.2816.Double a Number Represented as a Linked List
* No.2817.Minimum Absolute Difference Between Elements With Constraint
* No.2818.Apply Operations to Maximize Score

Author: yanglbme

Diff for: solution/2800-2899/2815.Max Pair Sum in an Array/README_EN.md

@@ -37,6 +37,12 @@ It can be shown that there are no other pairs with equal maximum digits, so the
 
 ## Solutions
 
+**Solution 1: Enumeration**
+
+First, we initialize the answer variable $ans=-1$. Next, we directly enumerate all pairs $(nums[i], nums[j])$ where $i \lt j$, and calculate their sum $v=nums[i] + nums[j]$. If $v$ is greater than $ans$ and the largest digit of $nums[i]$ and $nums[j]$ are the same, then we update $ans$ with $v$.
+
+The time complexity is $O(n^2 \times \log M)$, where $n$ is the length of the array and $M$ is the maximum value in the array.
+
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 ### **Python3**

Diff for: solution/2800-2899/2816.Double a Number Represented as a Linked List/README_EN.md

@@ -36,6 +36,12 @@
 
 ## Solutions
 
+**Solution 1: Reverse Linked List + Simulation**
+
+First, we reverse the linked list, then simulate the multiplication operation, and finally reverse the linked list back.
+
+Time complexity is $O(n)$, where $n$ is the length of the linked list. Ignoring the space taken by the answer linked list, the space complexity is $O(1)$.
+
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 ### **Python3**