NC15163逆序数

这其实就是一个归并排序

#include<iostream>
using namespace std;
#define IOS ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);

int a[100009], b[100009];
long long ans = 0;

void Merge_Sort(int l, int mid, int r) {
	int p1 = l, p2 = mid + 1;
	for (int i = l; i <= r;i++) {
		if ((p1 <= mid) && ((p2 > r) || (a[p1] <= a[p2]))) {
			b[i] = a[p1];
			p1++;
		}
		else {
			b[i] = a[p2];
			p2++;
			ans += mid - p1 + 1;
		}
	}
	for (int i = l;i <= r;i++) {
		a[i] = b[i];
	}
}

void dichotomy(int l, int r) {
	int mid = (l + r) >> 1;
	if (l < r) {
		dichotomy(l, mid);
		dichotomy(mid + 1, r);
	}
	Merge_Sort(l, mid, r);
}

int main() {
    IOS;
	int n;
	cin >> n;
	for (int i = 0;i < n;i++) {
		cin >> a[i];
	}
	dichotomy(0, n - 1);
	cout << ans << endl;
	return 0;
}

标签：p2,p1,NC15163,dichotomy,int,mid,++,序数
来源： https://blog.csdn.net/sunhongxuan666/article/details/113827934