上交oj1219 重要的逆序数对

题意：

https://acm.sjtu.edu.cn/OnlineJudge/problem/1219

思路：

在经典的归并排序求逆序数对算法基础上稍作修改。

实现：

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
const int MAXN = 200005;
int n, a[MAXN], tmp[MAXN];
long long cnt = 0;

void mergeSort(int * a, int left, int right, int * tmp)
{
    if (left >= right)
        return;
    int mid = (left + right) / 2;
    mergeSort(a, left, mid, tmp);
    mergeSort(a, mid + 1, right, tmp);
    int x = left;
    int y = mid + 1;
    int start = left;
    while (x <= mid && y <= right)
    {
        if (a[x] < a[y])
        {
            tmp[start++] = a[x++];
        }
        else
        {
            int pos = upper_bound(a + x, a + mid + 1, 2 * a[y]) - a;
            cnt += mid - pos + 1;
            tmp[start++] = a[y++];
        }
    }
    while(x <= mid)
    {
        tmp[start++] = a[x++];
    }
    while(y <= right)
    {
        tmp[start++] = a[y++];
    }
    for (int i = left; i <= right; i++)
    {
        a[i] = tmp[i];
    }
}
int main()
{
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%d", &a[i]);
    }
    mergeSort(a, 0, n - 1, tmp);
    cout << cnt << endl;
    return 0;
}