双指针分治算法求逆序数对

代码展示

#include<iostream>
#include<vector>
using namespace std;
typedef long long LL;

const int N = 100010;
int n;
int a[N], backup[N];
LL res;//处理大量数据的常用框架

LL merge_sort(int l, int r) {
	if (l >= r)return 0;
	int mid = (l + r) >> 1;//>> 1 表示将二进制数向右移动 1 位
	//相当于将数值除以 2 并向下取整
	res = (LL)merge_sort(l, mid) + merge_sort(mid + 1, r);
	int i = l, j = mid + 1, k = 0;
	while (i <= mid && j <= r) {
		if (a[i] <= a[j])backup[k++] = a[i++];
		else {
			backup[k++] = a[j++];
			res += mid - i + 1;
		}
	}
	while (i <= mid)backup[k++] = a[i++];
	while (j <= r)backup[k++] = a[j++];
	for (int i = l, j = 0; i <= r; i++, j++)a[i] = backup[j];
	return res;
}
int main() {
	scanf("%d", &n);
	for (int i = 1; i &l