题目描述

模板题：逆序对数量

注意用long！！！！！

C++代码

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 1;

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

int main(){
    int a[N];
    int n;
    cin >> n;
    for(int i = 0; i < n; i++)
        cin >> a[i];
    long res = merge_sort(a, 0, n - 1);
    cout << res << "\n";
    return 0;
}