hdoj 1514 Stars(树状数组类似求逆序)

星星的层数就是该星星左下（包括正左和正下）的数量。

因为题目说了是按y的升序给出，所以只要求前面x比它小的星星数就可以了。

每次读到一颗星星查询x的前缀和即为比自己小的星星数即层数，查完再update该点+1即可。

代码：

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
const int maxn = 4*1e4+5;
int n, tree[maxn], rec[maxn];

int lowbit(int i) { return i&(-i); }

void update(int pos, int val)
{
while(pos <= maxn)
{
tree[pos] += val;
pos += lowbit(pos);
}
}

int query(int pos)
{
int sum = 0;
while(pos)
{
sum += tree[pos];
pos -= lowbit(pos);
}
return sum;
}

int main(void)
{
while(cin >> n)
{
memset(tree, 0, sizeof(tree));
memset(rec, 0, sizeof(rec));
int t = n;
for(int i = 1; i <= n; i++)
{
int a, b;
scanf("%d%d", &a, &b);
a++;
rec[query(a)]++;
update(a, 1);
}
for(int i = 0; i < n; i++)
printf("%d\n", rec[i]);
}
return 0;
}

Stars

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)

Total Submission(s): 8765    Accepted Submission(s): 3489


[align=left]Problem Description[/align]
Astronomers often examine star maps where stars are represented by points on a plane and each star has Cartesian coordinates. Let the level of a star be an amount of the stars that are not higher and not to the right of the
given star. Astronomers want to know the distribution of the levels of the stars. 



For example, look at the map shown on the figure above. Level of the star number 5 is equal to 3 (it's formed by three stars with a numbers 1, 2 and 4). And the levels of the stars numbered by 2 and 4 are 1. At this map there are only one star of the level
0, two stars of the level 1, one star of the level 2, and one star of the level 3. 

You are to write a program that will count the amounts of the stars of each level on a given map.
 

[align=left]Input[/align]
The first line of the input file contains a number of stars N (1<=N<=15000). The following N lines describe coordinates of stars (two integers X and Y per line separated by a space, 0<=X,Y<=32000). There can be only one star
at one point of the plane. Stars are listed in ascending order of Y coordinate. Stars with equal Y coordinates are listed in ascending order of X coordinate.
 

[align=left]Output[/align]
The output should contain N lines, one number per line. The first line contains amount of stars of the level 0, the second does amount of stars of the level 1 and so on, the last line contains amount of stars of the level
N-1.
 

[align=left]Sample Input[/align]

5
1 1
5 1
7 1
3 3
5 5

 

[align=left]Sample Output[/align]

1
2
1
1
0

 

[align=left]Source[/align]
Ural Collegiate Programming
Contest 1999