归并排序求逆序对

Posted 2022-07-04 mch5201314

代码

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5 + 10;
int a[N],b[N];
ll ans = 0;
#define sc scanf
#define pr printf
#define rep(i,x,y) for(int i = x;i <= y;i++)
#define rvp(i,x,y) for(int i = y;i >= x;i--)
void merge(int l,int r)

    if(l == r) return ;
    int m = (l+r) >> 1;
    merge(l,m);
    merge(m+1,r);
    int p1=l,p2 = m + 1;
    int cur = l;
    while(p1 <= m || p2 <= r)
    
        if(p2 > r|| p1 <= m && a[p1] <= a[p2])
        
            b[cur++] = a[p1];
            p1++;
        
        else
            b[cur++] = a[p2];
            p2++;
            ans += (m - p1 + 1);
        
    
    rep(i,l,r) a[i] = b[i];


int main()

    int n;
    while(~sc("%d",&n))
    
        rep(i,1,n) sc("%d",&a[i]);
        ans = 0;
        merge(1,n);
        rep(i,1,n) pr("%d ",a[i]);
        puts("");
        pr("%lld\n",ans);
    
    return 0;