poj 2299 Ultra-QuickSort

Ultra-QuickSort

Time Limit: 7000MS		Memory Limit: 65536K
Total Submissions: 44601		Accepted: 16214

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

Source

Waterloo local 2005.02.05

分析：

求逆序对，因此利用归并排序，在排序过程中，记录逆序对的数目。

什么是逆序对

#include<iostream>
#include<cstdio>
using namespace std;
#define max 500000
int c[max+5];
int a[max+5];
long long count;
void Merge(int *a,int first,int last){
    int p,q,temp,mid;
    mid=(first+last)/2;
    temp=first;
    p=first;
    q=mid+1;
    while(p<=mid&&q<=last){
        if(a[p]>a[q]){
            count+=mid+1-p;
            c[temp++]=a[q++];
        }
        else{
            c[temp++]=a[p++];    
        }
    }
    while(q<=last){ 
            c[temp++]=a[q++];
    }
    q--;
    while(p<=mid){
            //count+=q-mid;
            c[temp++]=a[p++];
    }
    int i=first;
    for(;i<=last;i++){
        a[i]=c[i];
    }
}
void MergeSort(int *a,int first,int last){
    if(first==last){
        return;
    }
    int mid=(first+last)/2;
    MergeSort(a,first,mid);
    MergeSort(a,mid+1,last);
    Merge(a,first,last);
}
int main(){
    int n;
    while(scanf("%d",&n)&&n){
        count=0;
        int i=0;
        for(;i<n;i++){
             scanf("%d",&a[i]);
        }
        MergeSort(a,0,n-1);
         cout<<count<<endl;
    }
    //cout<<1<<endl;
    return 0;
}