1. 思想
分治法:
- 選擇中間點
- 遞歸排序左邊、右邊
- 歸併
2. 模板
#include <iostream>
using namespace std;
const int N = 1e6+10;
int arr[N],tmp[N];
void mergeSort(int *arr, int l, int r)
{
if(l >= r) return ;
int mid = (l+r)>>1; //1. 選擇中間點
mergeSort(arr, l, mid); //2. 遞歸排序左邊、右邊
mergeSort(arr, mid+1, r);
int k = 0, i = l, j = mid + 1;
while(i<=mid && j<=r) //3. 歸併
{
if(arr[i] < arr[j])
tmp[k++] = arr[i++];
else
tmp[k++] = arr[j++];
}
while(i <= mid) tmp[k++] = arr[i++];
while(j <= r) tmp[k++] = arr[j++];
for(int i=0, j=l; j<=r; i++,j++) arr[j] = tmp[i];
}
int main()
{
int n;
scanf("%d", &n);
for(int i=0; i<n; i++)
scanf("%d", &arr[i]);
mergeSort(arr, 0, n-1);
for(int i=0; i<n; i++)
printf("%d ", arr[i]);
return 0;
}
3. 應用
- 求逆序對
#include <iostream>
using namespace std;
const int N = 1e6+10;
int arr[N], tmp[N];
long long mergeSort(int *arr, int l, int r)
{
if(l >= r) return 0;
int mid = (l+r)>>1;
long long ret = mergeSort(arr, l, mid) + mergeSort(arr, mid+1, r);
int k = 0, i = l, j = mid+1;
while(i<=mid && j<=r)
{
if(arr[i] <= arr[j])
tmp[k++] = arr[i++];
else{
ret += mid-i+1;
tmp[k++] = arr[j++];
}
}
while(i <= mid) tmp[k++] = arr[i++];
while(j <= r) tmp[k++] = arr[j++];
for(int i=0, j=l; j<=r; i++,j++) arr[j] = tmp[i];
return ret;
}
int main()
{
int n;
scanf("%d", &n);
for(int i=0; i<n; i++) scanf("%d", &arr[i]);
printf("%ld\n", mergeSort(arr, 0, n-1));
//for(int i=0; i<n; i++) cout<<arr[i]<<" ";
return 0;
}
-
統計時機:
當arr[i] > arr[j]時,(mid-i+1)個數可與arr[j]構成逆序對。
-
注:本系列參考:AcWing。。。