A. Eshag Loves Big Arrays
Because can choose any subsequence. So, for some number x which is bigger than the minimum number of the array. This x will be nearby one of minimum number earlier or later. And this x will be removed then.
B. Sifid and Strange Subsequences
First, choose all numbers which is less or equal than 0 is a choice.
Then we consider if we choose some positive numbers.
We can find that: at most we can pick up one positive number. And so, of course, it is better to choose smaller number. So, we choose the minimum positive value.
So, for the remained numbers, we choose the number from big to small one by one. And just need make sure the absolute value of the difference between two adjacent numbers is more or equal to the positive number.
Finally, we choose the max value between "all number less/equal to 0" and "one positive number".
C. Parsa's Humongous Tree
Very classical problem of DP on the tree.
D. Kavi on Pairing Duty
Just solve the case which n=4 manually. 70% of cases is obviously. The only hidden case is when n=4, can get another solution which is repeat the n=2 twice.