《树状数组求逆序对数》

#include<stdio.h>
#include<string.h>
#define maxn 1005
int tree[maxn]; int lowbit(int x) { return x&(-x); } void update(int pos,int num) { while(pos<=maxn) {
        tree[pos]+=num;
        pos+=lowbit(pos); } } int sum(int pos) { int ans=0; while(pos>0) {
        ans+=tree[pos];
        pos-=lowbit(pos); } return ans; } int main() { int t,i,num,ans; while(~scanf("%d",&t)) {
        memset(tree,0,sizeof(tree));
        ans=0; for(i=1;i<=t;i++) {
            scanf("%d",&num);
            update(num,1);
            ans+=i-sum(num); }
        printf("%d\n",ans); } return 0; 
}
 
http://acm.hdu.edu.cn/showproblem.php?pid=2689 

  
  
  
  

   
   
   
   Problem Description
  
  
  
  
  
  
  
  

   
   
   
   You want to processe a sequence of n distinct integers by swapping two adjacent sequence elements until the sequence is sorted in ascending order. Then how many times it need.
For example, 1 2 3 5 4, we only need one operation : swap 5 and 4.

  
  
  
  
  
  
  
  

   
   
   
    
  
  
  
  

  
  
  
  

   
   
   
   Input
  
  
  
  
  
  
  
  

   
   
   
   The input consists of a number of test cases. Each case consists of two lines: the first line contains a positive integer n (n <= 1000); the next line contains a permutation of the n integers from 1 to n.
  
  
  
  
  
  
  
  

   
   
   
    
  
  
  
  

  
  
  
  

   
   
   
   Output
  
  
  
  
  
  
  
  

   
   
   
   For each case, output the minimum times need to sort it in ascending order on a single line.
  
  
  
  
  
  
  
  

   
   
   
    
  
  
  
  

  
  
  
  

   
   
   
   Sample Input
  
  
  
  
  
  
  
  

   
   
   
   
    
    
    
    

     
     
     
     3
1 2 3
4 
4 3 2 1 
    
    
    
    
  
  
  
  
  
  
  
  

   
   
   
    
  
  
  
  

  
  
  
  

   
   
   
   Sample Output
  
  
  
  
  
  
  
  

   
   
   
   
    
    
    
    

     
     
     
     0
6
    
    
    
    
  
  
  
  
  
  
  
  

   
   
   
    
  
  
  
  

  
  
  
  

   
   
   
   Author
  
  
  
  
  
  
  
  

   
   
   
   WhereIsHeroFrom