(unsolved) a hard problem

Given N non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

It’easy.

So you can inc some bar,but the sum of the height you inc no more than M.

Now,maximize the answer.

1 <= N <= 1000

0 <= M <= 1000

0 <= ai <= 1000

For example,
n = 3,m = 0

A : [1 , 0 , 1]

answer : 1

n = 3, m = 2

A : [1 , 0 , 1]

answer : 2 (you can inc a[1] and a[3])

Please give the link of the question…

To ensure it is not from any ongoing contest

Not link,just some one ask for help,all of them are integer.
I hava a idea but it’s O(nm^2)

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