POJ2299：Ultra-QuickSort——题解

http://poj.org/problem?id=2299

题目大意：给一串数，求其按照两两交换排序最少排几次。

求逆序对裸题，不建议用数据结构（因为需要离散化）

#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstring>
#include<iostream>
using namespace std;
typedef long long ll;
ll n,a[500001],t[500001];
ll ans = 0;
void msort(int x,int y)
{
    if(x==y) return;
    int mid = (x + y) >>1;
    msort(x,mid);
    msort(mid+1,y);
    int l = x, r = mid+1, j = x;
    while(l<=mid && r<=y)
    {
    if(a[l]<=a[r]){
        t[j] = a[l]; j++;l++;
    }
    else{
        t[j] = a[r]; r++;j++; 
        ans += mid-l+1;
    }
    }
    while(l<=mid){
    t[j] = a[l]; j++;l++;
    }
    while(r<=y){
    t[j] = a[r]; r++;j++;
    }
    for(int i=x;i<=y;i++)a[i] = t[i];
    return;
}
int main(){
    while(scanf("%lld",&n)!=EOF){
    if(!n)return 0;
    ans=0;
    for(int i=1;i<=n;i++){
        scanf("%lld",&a[i]);
    }
    msort(1,n);
    printf("%lld
",ans);
    }
    return 0;
}