2015 HUAS Summer Trainning #4 A

Description

In this problem, you have to analyze a particular sorting algorithm. The algorithm processes a sequence of n distinct integers by swapping two adjacent sequence elements until the sequence is sorted in ascending order. For the input sequence 9 1 0 5 4 ,Ultra-QuickSort produces the output 0 1 4 5 9 .Your task is to determine how many swap operations Ultra-QuickSort needs to perform in order to sort a given input sequence.

Input

The input contains several test cases. Every test case begins with a line that contains a single integer n < 500,000 -- the length of the input sequence. Each of the the following n lines contains a single integer 0 ≤ a[i] ≤ 999,999,999, the i-th input sequence element. Input is terminated by a sequence of length n = 0. This sequence must not be processed.

Output

For every input sequence, your program prints a single line containing an integer number op, the minimum number of swap operations necessary to sort the given input sequence.

Sample Input

5
9
1
0
5
4
3
1
2
3
0

Sample Output

6

0

题目大意：求逆序数的个数；

解题思路：因为有时间限制，不能有2层循环来找。需要用到归并排序。

代码：

#include<iostream>
#include<algorithm>
using namespace std;
const long long  maxn=500000+10;
int a[maxn];
long long  paisu(int x,int y)
{
    if(x>=y) return 0;
    long long  cnt=0;
    int m=x+(y-x)/2;
    cnt+=paisu(x,m);
    cnt+=paisu(m+1,y);
    int p2=m+1;
    for(int p1=x;p1<=m;p1++)
    {
        for(int j=p2;j<=y;j++)
        {
            if(a[p1]<a[j])
                break;
            else p2++;
        }
        cnt+=p2-m-1;
    }
    sort(a+x,a+y+1);
    return cnt;

}
int main()
{
    int i;
    int n;
    while(cin>>n&&n)
    {
        for(i=0;i<n;i++)
            cin>>a[i];
        cout<<paisu(0,n-1)<<endl;
    }
    return 0;
}