应用归并排序（求逆序数）

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
大意：求冒泡排序交换的个数，也就是求逆序数，套一下模板

#include<cstring>

#include<cstdio>

#include<algorithm>

const int MAX = 510000;

int a[MAX],temp[MAX];

int ans = 0;

void mergesort(int *a,int first,int mid,int last,int *temp)

{

   int i = first,j = mid + 1;

   int  k = first;

    while( i <= mid && j <= last){

          if(a[i] <= a[j])

          temp[k++] = a[i++];

          else {

            ans += j - k;

           temp[k++] = a[j++];

          }

    }

    while( i <= mid)

        temp[k++] = a[i++];

    while( j <= last)

        temp[k++] = a[j++];

    for(int i = first;i <= last;i++)

        a[i] = temp[i];

}

void mergearray(int *a,int first,int last,int *temp)

{

     if(first < last){

            int mid = (first + last) >> 1;

            mergearray(a,first,mid,temp);

            mergearray(a,mid+1,last,temp);

            mergesort(a,first,mid,last,temp);

     }

}

int main()

{

    int n;

    scanf("%d",&n);

    for(int i = 1; i <= n; i++)

    scanf("%d",&a[i]);

    mergearray(a,1,n,temp);

    printf("%d",ans);

  return 0;

}

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