C++
$\color{#cc33ff}{— > 算法基础课题解}$
$\color{gold}{— > 蓝桥杯辅导课题解}$
思路:
归并排序
$在归并过程中记录逆序对数量res$
$\color{#ff33ff}{图解:}$
#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 1e5 + 10;
typedef long long ll;
int n;
int q[N], tmp[N];
ll merge_sort(int l, int r) {
if (l >= r) return 0;
int mid = l + r >> 1;
ll res = merge_sort(l, mid) + merge_sort(mid + 1, r);
int k = 0, i = l, j = mid + 1;
while (i <= mid && j <= r)
if (q[i] <= q[j]) tmp[k ++] = q[i ++];
else {
tmp[k ++] = q[j ++];
res += mid - i + 1; // 记录
}
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() {
cin >> n;
for (int i = 0; i < n; i ++) cin >> q[i];
cout << merge_sort(0, n - 1);
return 0;
}