788. Number of Reversed Pairs

Idea:
Use the method of merging to count the number in merging and sorting

Code:

# include<iostream>
# include<cstdio>
# include<cstring>
# include<algorithm>
using namespace std;

typedef long long LL;

const int N = 100010;
int q[N],tmp[N];
int n;
LL res;

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

int main()
{
    
    
    scanf("%d",&n);
    for(int i = 0;i < n;i++)
    {
    
    
        scanf("%d ",&q[i]);
    }
    
    cout << merge_sort(0,n - 1) << endl;
    
    return 0;
}