归并排序模板

逆序对永远只会树状数组写法OrzOrzOrz……

#include <stdio.h>

const int _N = 12000;

int A[_N], B[_N], cnt;

void Merge(int l, int r)
{
    int i, j, k, mid;
    mid = l+r >> 1;
    i = k = l, j = mid+1;
    while (i <= mid && j <= r)
        if (A[i] <= A[j]) B[k++] = A[i++];
        else B[k++] = A[j++], cnt += mid-i+1;
    while (i <= mid) B[k++] = A[i++];
    while (j <= r) B[k++] = A[j++];
    for (i = l; i <= r; ++i) A[i] = B[i];
    return;
}

void MergeSort(int l, int r)
{
    if (l >= r) return;
    int mid = l+r >> 1;
    MergeSort(l, mid), MergeSort(mid+1, r);
    Merge(l, r);
    return;
}

int main()
{
    int i, N;
    scanf("%d", &N);
    for (i = 1; i <= N; ++i) scanf("%d", &A[i]);
    MergeSort(1, N);
    for (i = 1; i <= N; ++i) printf("%d ", A[i]);
    printf("
NiXuDuiShu: %d
", cnt);
    return 0;
}