归并排序-分治

第一步

将数组q[N]一分为二,一个无序的数组成为两个数组

第二步

合二为一,将两个有序数组合并成为一个有序数组

#include 
using namespace std;
const int N=100010;
int q[N],res[N];

void merge_sort(int q[],int l,int r)
{
    if(l>=r)return;
    int mid=(l+r)/2;
    merge_sort(q,l,mid),merge_sort(q,mid+1,r);
    int i=l,j=mid+1,k=0;
    while(i<=mid&&j<=r)
    {
        if(q[i]<=q[j])res[k++]=q[i++];
        else res[k++]=q[j++];
    }
    while(i<=mid)res[k++]=q[i++];
    while(j<=r)res[k++]=q[j++];
    for(int i=l,j=0;i<=r;i++,j++)q[i]=res[j];
}

int main()
{
    int n;
    cin>>n;
    for(int i=0;i>q[i];
    merge_sort(q,0,n-1);
    for(int i=0;i 
练习：逆序对的数量
 
#include 
using namespace std;
const int N=100010;
typedef long long LL;
int q[N],res[N];

LL merge_sort(int q[],int l,int r)
{
    if(l>=r)return 0;
    int mid=(l+r)/2;
    LL sum=merge_sort(q,l,mid)+merge_sort(q,mid+1,r);
    int i=l,j=mid+1,k=0;
    while(i<=mid&&j<=r)
    {
        if(q[i]<=q[j])res[k++]=q[i++];
        else
        {
            sum+=mid-i+1;
            res[k++]=q[j++];
        }
    }
    while(i<=mid)res[k++]=q[i++];
    while(j<=r)res[k++]=q[j++];
    for(int i=l,j=0;i<=r;i++,j++)q[i]=res[j];
    return sum;
}

int main()
{
    int n;
    cin>>n;
    for(int i=0;i>q[i];
    cout<