sebi420p's blog

Can 2D BIT be built in O(n^2)?
By sebi420p, 5 years ago, In English

Knowing the fact that 1D Fenwick Trees can be built in linear time, I am assuming that the 2D version can be built in quadratic time. I didn't manage to find anything about this. Can you help me with some links/ideas?

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5 years ago, # |
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For every updation in linear Fenwick tree we require O(log N ) time complexity.It takes Nlog(N) for linear or 1-D. sebi420p How can we built in just O(N) .

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    5 years ago, # ^ |
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    Actually, we can build a bit in O(n). Every index in a bit indicates the sum of the range [i — lowbit(i), i]. So, if you build a prefix sum array beforehand, you can use it to build the bit in O(n).

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    5 years ago, # ^ |
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    You can check this link

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5 years ago, # |
  Vote: I like it +37 Vote: I do not like it 
	for(int i=1;i<=n;++i)
	for(int j=1;j<=n;++j)c[i][j]=a[i][j];
	for(int i=1;i<=n;++i)
	for(int j=1;j<=n;++j)
	if(j+(j&-j)<=n)c[i][j+(j&-j)]+=c[i][j];
	for(int i=1;i<=n;++i)
	for(int j=1;j<=n;++j)
	if(i+(i&-i)<=n)c[i+(i&-i)][j]+=c[i][j];
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5 years ago, # |
  Vote: I like it +10 Vote: I do not like it

Why is he downvoted, it's a good question.

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