问题
I am trying to solve this problem in UVA with Binary Indexed Tree:
Problem H
Ahoy, Pirates!
Input: Standard Input
Output: Standard Output
In the ancient pirate ages, the Pirate Land was divided into two teams of pirates,
namely, the Buccaneer and the Barbary pirates. Each pirate’s team was not fixed,
sometimes the opponent pirates attacked and he was taken away to the other pirate team.
All on a sudden a magician appeared in the Pirate Land, where he was making transition
of pirates from their team to other team at his own will. Of course, handy spells
were used. The process of changing team was known as mutating.
There were N pirates and all of the pirates have a unique id from 0 to N-1.
The great magician could mutate a bunch of pirates with consecutive id’s to
another one.
Suppose there were 100 pirates in the pirate land and all of them were
Barbary pirates, then the magician could cast a spell to change pirates with
id’s from 10 to 33 to Buccaneer pirates. Then the whole pirate land would
have 24 Buccaneer and 76 Barbary pirates.
The magician was very fast casting the spell. Once, God started to dislike this.
God had favor for the Buccaneer pirates and God asked the magician, “Tell me,
how many of the pirates of index from 2 to 30 are Buccaneer pirates?”. Now the
magician was puzzled as he was only efficient in casting spells, not in counting J
Being clever enough, the magician captured a clever man from the Earth Land.
And unfortunately it’s you! Now you have to answer the God’s questions.
Input
The first line of input will contain number of test cases T.
For each test case:
The first part of the description will be of the pirate land.
There could be up to N (1<=N<=1024000) pirates. Each pirate is either
assigned to Buccaneer or Barbary Pirate. Buccaneer pirates are described
by ‘1’ (ONE) and Barbary pirates are described by ‘0’ (ZERO). You have to
build a string of the pirates’ description. Each case starts with an integer
M (M<=100), where M pair lines follow. In each pair of lines (we call it a set),
first has an integer T (T <= 200) and next one has a nonempty string Pirates
(consisting of 0 and 1, 0 for Barbary, 1 for Buccaneer, has maximum length of 50).
For each pair concatenate the string Pirates, T times. Concatenate all the
resulting M sets of strings to build the pirate description. The final
concatenated string describes the pirates from index 0 to end (N-1 for N pirates).
Now the next part of the input will contain queries.
First line of next part has an integer Q describing number of queries.
Each subsequence Q (1<=Q<=1000) lines describe each query. Each query has a
string F or E or I or S and two integers, a and b denoting indexes.
The meaning of the query string are follows:
F a b, means, mutate the pirates from index a to b to Buccaneer Pirates.
E a b, means, mutate the pirates from index a to b to Barbary Pirates.
I a b, means, mutate the pirates from index a to b to inverse pirates.
S a b, means, “God’s query” God is asking a question:
“Tell me, how many Buccaneer pirates are there from index a to b?”
(a <= b, 0 <= a < n, 0 <= b < n, index range are inclusive)
Output
For each test print the case number as the sample output suggests.
Then for each of “God’s query”, output the query number, colon (:) and
a space and the answer to the query as the sample suggest.
╔══════════════╦═════════════════════════╗
║ Sample Input ║ Output for Sample Input ║
╠══════════════╬═════════════════════════╣
║ 2 ║ Case 1: ║
║ 2 ║ Q1: 5 ║
║ 5 ║ Q2: 1 ║
║ 10 ║ Case 2: ║
║ 2 ║ Q1: 0 ║
║ 1000 ║ ║
║ 5 ║ ║
║ F 0 17 ║ ║
║ I 0 5 ║ ║
║ S 1 10 ║ ║
║ E 4 9 ║ ║
║ S 2 10 ║ ║
║ 3 ║ ║
║ 3 ║ ║
║ 1 ║ ║
║ 4 ║ ║
║ 0 ║ ║
║ 2 ║ ║
║ 0 ║ ║
║ 2 ║ ║
║ I 0 2 ║ ║
║ S 0 8 ║ ║
╚══════════════╩═════════════════════════╝
Explanation:
Case1:
The pirate land is as follows (N = 18)
101010101010001000
Before God’s first query it was as follows
000000111111111111
Case 2:
The pirate land is as follows (N=9)
111000000
It's about incrementing or decrementing a range of values to 1 or 0, and query is as usual in fenwick tree.
I know How to update at a specific position in a tree.
When updating a range I just used a loop for every element in that range to update to that 1 or 0. But Its taking too much time.
Is there any other way to range update in a fenwick tree?
Actually it's an array of 1' and 0's with nothing else, and the update procedure only includes updating a range [a,b] to 1,0 or inverting. Here's my code:
#include <iostream>
#include <vector>
#include <stdio.h>
using namespace std;
typedef vector<int> vi;
class bit{
public:
bit(int n,string strt):s(strt){ ftBuc.assign(n+2,0); }
void adjust(int k,int v){
if(v=='1'&&s[k] == '1')
return;
else if(v=='0' && s[k] == '0')
return;
else if(v=='1'&&s[k] == '0')
{ s[k] = '1'; for(;k<(int)ftBuc.size();k += leastSig1(k)) ftBuc[k]+=1; }
else if(v=='0' && s[k] == '1' )
{ s[k] = '0'; for(;k<(int)ftBuc.size();k += leastSig1(k)) ftBuc[k]-=1; }
}
int rsq(int a,int b) { return rsq(b)-(a==1?0:rsq(a-1));}
string s;
int rsq(int b) { int sum = 0; for(; b ; b-=leastSig1(b)) sum+=ftBuc[b]; return sum;}
vi ftBuc;
int leastSig1(int i) {return i&(-i);}
};
int main()
{
int t;
cin>>t;
for(int test=1;test<=t;++test)
{
int m;
cin>>m;
string s;
while(m--)
{
int T;
cin>>T;
string temp;
cin>>temp;
while(T--)
s.append(temp);
}
int n = s.size();
bit ft(n+2,string(n+2,'0'));
for(int i = 1;i<=n;++i)
ft.adjust(i,s[i-1]);
int q;
cin>>q;
int asks = 1;
printf("Case %d:\n",test);
while(q--)
{
char c;
int i,j;
cin>>c>>i>>j;
if(c=='F')
{
for(int k=i+1;k<=j+1;++k)
ft.adjust(k,'1');
}
else if(c=='E')
{
for(int k=i+1;k<=j+1;++k)
ft.adjust(k,'0');
}
else if(c == 'I')
{
for(int k=i+1;k<=j+1;++k)
{
if(ft.s[k] == '1')
ft.adjust(k,'0');
else
ft.adjust(k,'1');
}
}
else if(c=='S')
printf("Q%d: %d\n",asks++,ft.rsq(i+1,j+1));
}
}
}
回答1:
A binary indexed tree (aka Fenwick tree) is an efficient way of representing a frequency table in order to be able to quickly extract cumulative frequencies and uses N bins to hold N frequency values.
However, for this problem I would recommend a segment tree instead. A segment tree has a similar structure to a binary indexed tree except it typically uses 2N bins instead.
Fenwick tree
As with the Fenwick tree, it is helpful to think about each node in the tree as being responsible for a set of indices. Here is an illustration of the responsibilities for nodes in a Fenwick tree of size 8:
Index Responsibilities
8 8
7 7 8
6 6 8
5 5 6 8
4 4 8
3 3 4 8
2 2 4 8
1 1 2 4 8
This means that, for example, the Fenwick entry A[4] will be responsible for indices 1,2,3,4 and A[4] will contain the total frequency F[1]+F[2]+F[3]+F[4].
Segment Tree
In contrast, the responsibilities in a segment tree will look like:
Index Responsibilities
8 1 3 7 15
7 1 3 7 14
6 1 3 6 13
5 1 3 6 12
4 1 2 5 11
3 1 2 5 10
2 1 2 4 9
1 1 2 4 8
Lazy Update
So why have we done this?
The reason is that we now have a separate entry responsible for each subdivided range, and this lets us use lazy update.
The idea with lazy update is that when you wish to update a range of values you only need to recurse into the segment tree and subdivide until you reach an interval that is totally contained within the range to be updated. Once you reach such an interval you do two things:
- update the values for that interval
- store information at that node to indicate that the segments underneath still need to be updated
Note that during the recursion you also need to push down up any lazy operations you encounter.
For example code, try searching for "segment tree" and "lazy propagation".
Complexity
Segment trees with lazy propagation will cost O(log(N)) for both queries and updates of any range of values.
回答2:
A BIT can be operated in one of 3 modes:
- Point updates and range queries
- Range updates and point queries
- Range updates and range queries
I've explained range updates with BIT and provided implementation here: http://kartikkukreja.wordpress.com/2013/12/02/range-updates-with-bit-fenwick-tree/
来源：https://stackoverflow.com/questions/18065238/how-to-do-a-range-update-in-binary-indexed-tree-or-fenwick-tree