1 package com.aixuexi.contact;
2
3 public class Mergesort {
4
5 public static void mergeSort(int arr[]) {
6 if(arr == null || arr.length <= 2) {
7 return ;
8 }
9 mergeSort(arr,0,arr.length-1);
10 }
11 public static void mergeSort(int arr[],int L,int R) {
12
13 if(L == R) {
14 return ;
15 }
16 int mid = (L + R ) / 2;
17 mergeSort(arr,L,mid);
18 mergeSort(arr,mid+1,R);
19 merge(arr,L, mid, R);
20 }
21 public static void merge(int arr[],int L,int mid,int R) {
22 int arr1[] = new int[R - L + 1];
23 int i = 0;
24 int p1 = L;
25 int p2 = mid + 1;
26 while(p1 <= mid && p2 <= R) {
27 arr1[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
28 }
29
30 while (p1 <= mid) {
31 arr1[i++] = arr[p1++];
32 }
33 while (p2 <= R) {
34 arr1[i++] = arr[p2++];
35 }
36 for (i = 0; i < arr1.length; i++) {
37 arr[L + i] = arr1[i];
38 }
39
40 }
41
42 public static void main(String[] args) {
43 int arr[] = new int[] {1,4,8,3,7,3,43,43,6,8,87,9,4,67};
44 mergeSort(arr);
45 for(int i = 0; i < arr.length; i++) {
46 System.out.println(arr[i]);
47 }
48 }
49 }
package com.aixuexi.contact;
/* 2019/7/16 09点25分
*
* 总结： 1个问题行数 --28 行
* 注意点 38行
*
* return mergeSort(arr, l, mid) + mergeSort(arr, mid + 1, r) + merge(arr, l, mid, r);
* res += arr[p1] < arr[p2] ? (r - p2 + 1) * arr[p1] : 0; //算下标
*/
public class SmallSum {
public static int smallSum(int[] arr) {
if (arr == null || arr.length < 2) {
return 0;
}
return mergeSort(arr, 0, arr.length - 1);
}
public static int mergeSort(int[] arr, int l, int r) {
if (l == r) {
return 0;
}
int mid = l + ((r - l) >> 1);
// mergeSort(arr, l, mid); // 0
// mergeSort(arr, mid + 1, r); // 0
// int ret = merge(arr, l, mid, r); //ret = 9
return mergeSort(arr, l, mid) + mergeSort(arr, mid + 1, r) + merge(arr, l, mid, r);
}
public static int merge(int[] arr, int l, int m, int r) {
int[] help = new int[r - l + 1];
int i = 0;
int p1 = l;
int p2 = m + 1;
int res = 0;
while (p1 <= m && p2 <= r) {
res += arr[p1] < arr[p2] ? (r - p2 + 1) * arr[p1] : 0; //算下标
help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
}
while (p1 <= m) {
help[i++] = arr[p1++];
}
while (p2 <= r) {
help[i++] = arr[p2++];
}
for (i = 0; i < help.length; i++) {
arr[l + i] = help[i];
}
return res;
}
public static void main(String[] args) {
int arr[] = new int[] {1,3,4,2,5};
int res = smallSum(arr);
System.out.println(res);
// for(int i = 0; i < arr.length; i++) {
// System.out.println(arr[i]);
// }
}
}
