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Divide array in three parts
By maxifier, history, 3 years ago, In English

I was solving this problem. While solving this problem i encountered a sub-problem.

We have an array of n length and we want to divide it into three parts such that after dividing the array sum of individual arrays should be as close to the power of 2 greater than or equal to the sum as possible.

More formally, let say sum1, sum2, sum3 are the sums of subarrays. Our answer is minimum possible value of (p1 — sum1) + (p2 — sum2) + (p3 — sum3). Here, p1, p2, p3 are the powers of two.

For Example : let say our array is [4, 1, 1, 2, 3, 1]

-> If we divide this array as [4, 1], [1, 2], [3, 1] then our answer is (8 — 5) + (4 — 3) + (4 — 4) = 0.

-> If we divide this array as [4], [1, 1, 2], [3, 1] the our answer is (4 — 4) + (4 — 4) + (4 — 4) = 0 which is optimal in this case.

Constraints: 1<= n <= 1e5, 1 <= ai <=n

I would be grateful if you help me to solve this Problem.

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Comments Comments (6)
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3 years ago, # |
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isn't that the task itself?

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3 years ago, # |
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your problem can be solved like this.

let call nxt[x] : the next power of 2 greater than or equal x.

if we split the array into three blocks lets say sum of blocks are x,y,z.

the task is to minimize nxt[x]+nxt[y]+nxt[z]-x-y-z.

so notice that if we fix x and increased y by value a without change nxt[y] then:

it will be nxt[x]+nxt[y]+nxt[z]-x-(y+a)-(z-a)

as you can see we said that nxt[y] didn't change so it will not make answer worse but after decreasing z it's possible for nxt[z] to decrease

so lets try to fix x as any point (n point total) then for each power of 2 (call it p)(there logn different power of 2) binary search for a last point where the sum of second block (called y) will be smaller than or equal p (you can pre process the array by a prefix sum so you can binary search) so the total complexity is O(n*log^2(n))

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    3 years ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    another optimization to work only in n*log(n) is to use different pointer for each power of 2 so logn pointer total, then while increasing x (the first block) move the pointers forward so each pointer will change at most n times so n*log(n) for pointers walk and also n*log(n) for trying each power of 2 so the total is ( 2*n*log(n) )

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