ll qans;           // 記錄答案 
int date[maxn];    // 數據數組 
int tarray[maxn];  // 歸並排序的中間數組 

// POJ - 2299     SGU - 180
void merge(int arr[], int left, int right, int tarr[]) 
{
    if (left>=right) return ;
    int m = (left+right) >> 1;
    merge(arr, left, m, tarr);
    merge(arr, m+1, right, tarr);
    
 
int i,j, cnt;
    i = left;
    j = m+1;
    cnt = 0;
    while (i<=m && j<=right) {
        if (arr[i] <= arr[j]) 
            tarr[cnt++] = arr[i++];
        else {
            qans += (ll)m - i + 1;  // 求逆序對. 
            tarr[cnt++] = arr[j++]; 
        }
    }
    while (i<=m) tarr[cnt++] = arr[i++];
    
 
while (j<=right) tarr[cnt++] = arr[j++];
    for (i=0; i<cnt; ++i) arr[left++] = tarr[i]; 
}

歸並排序(逆序對數)