51nod 1019 逆序数

#include <iostream>

const int MAX = 5e4 + 5;
int res;
int arr[MAX];
int arr2[MAX];

void mergesort(int a[], int tmp[], int l, int r)
{
if (l == r)
return ;
int m = (l + r) >> 1;
mergesort(a, tmp, l, m);
mergesort(a, tmp, m + 1, r);
for (int i = l; i <= r; ++i)
tmp[i] = a[i];
int i1 = l, i2 = m + 1;
for (int curr = l; curr <= r; ++curr)
{
if (i1 == m + 1)
a[curr] = tmp[i2++];
else if (i2 > r)
a[curr] = tmp[i1++];
else if (tmp[i1] <= tmp[i2])
a[curr] = tmp[i1++];
else
{//当左边的子序列出现一个数大于右边的子序列的时候，左边子序列末尾数的位置减去当前左边这个数的位置加1就得到了有多少逆序对了
//  std::cout << tmp[i1] << " " << tmp[i2] << std::endl;
res += (m - i1 + 1);
a[curr] = tmp[i2++];
}
}
}

int main()
{
int num;
std::cin >> num;
for (int i = 0; i != num; ++i)
std::cin >> arr[i];
mergesort(arr, arr2, 0, num - 1);

std::cout << res << std::endl;

//for (int i = 0; i != num; ++i)
//  std::cout << arr[i] << ' ';

return 0;

}