逆序对
逆序数的定义:如果 i < j 且A[i] > A[j].则A[i]和A[j]即为逆序数对
给一列数<a[1],a[2],…,a[n]>,求它的逆序对数,即有多少个有序对(i.j),使得i<j且a[i]<a[j]
在归并排序的基础上记上了一个逆序对个数的统计
ans += (mid - i + 1)
void Merge(int arr[], int left, int mid, int right) {
int tmp[maxn];
int i = left, j = mid + 1, k = left;
while (i < mid + 1 && j < right + 1) {
if (arr[i] < arr[j]) {
tmp[k++] = arr[i++];
}
else {
ans += (mid - i + 1); //添加此句进行个数统计
tmp[k++] = arr[j++];
}
}
while (i<mid + 1)
tmp[k++] = arr[i++];
while (j<right + 1) {
tmp[k++] = arr[j++];
}
for (int i = left; i<right + 1; i++)
arr[i] = tmp[i];
return;
}
void MergeSort(int arr[], int left, int right) {
if (left < right) {
int mid = (left + right) / 2;
MergeSort(arr, left, mid);
MergeSort(arr, mid + 1, right);
Merge(arr, left, mid, right);
}
return;
}