【算法】归并排序（可求逆序数或逆序对）

int L[100010];
int R[100010];

void merge(int p[],int l,int mid,int r){
int len1 = mid - l;
int len2 = r - mid;
for(int i = 0;i < len1; i++){
L[i] = p[l+i];
}
L[len1] = INF;
for(int i = 0;i < len2; i++){
R[i] = p[mid+i];
}
R[len2] = INF;

int con1 = 0;
int con2 = 0;
for(int i = l;i < r; i++){
if(L[con1] < R[con2]){
p[i] = L[con1];
con1++;
}else if(L[con1] > R[con2]){
p[i] = R[con2];
con2++;
}else{
p[i] = L[con1];
con1++;
}
}
}

void mergeSort(int p[],int l,int r){
if(r - l > 1){
int mid = (l+r)/2;
mergeSort(p,l,mid);
mergeSort(p,mid,r);

merge(p,l,mid,r);
}
}