在一个数组中，一个数左边比它小的数的总和，叫数的小和，所有数的小和累加起来，叫数组小和，求数组小和，可以利用归并排序
package com.harrison.class03;
public class Code02_SmallSum {
public static int smallSum(int[] arr) {
if (arr == null || arr.length < 2) {
return 0;
}
return process(arr, 0, arr.length - 1);
}
public static int process(int[] arr, int L, int R) {
if (L == R) {
return 0;
}
int mid = L + ((R - L) >> 1);
return process(arr, L, mid)+
process(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]?arr[p1]*(R-p2+1):0;
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 int comparator(int[] arr) {
int res=0;
if(arr==null || arr.length<2) {
return 0;
}
for(int i=1; i<arr.length; i++) {
for(int j=0; j<i; j++) {
res+=arr[j]<arr[i]?arr[j]:0;
}
}
return res;
}
public static int[] generateRandomArray(int maxSize, int maxValue) {
int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
for (int i = 0; i < arr.length; i++) {
arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
}
return arr;
}
public static int[] copyArray(int[] arr) {
if (arr == null) {
return null;
}
int[] res = new int[arr.length];
for (int i = 0; i < arr.length; i++) {
res[i] = arr[i];
}
return res;
}
public static void printArray(int[] arr) {
if (arr == null) {
return;
}
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i] + " ");
}
System.out.println();
}
public static void main(String[] args) {
int testTimes=1000000;
int maxSize=100;
int maxValue=100;
boolean succeed=true;
for(int i=0; i<testTimes; i++) {
int[] arr1=generateRandomArray(maxSize, maxValue);
int[] arr2=copyArray(arr1);
if(smallSum(arr1)!=comparator(arr2)) {
succeed=false;
printArray(arr1);
printArray(arr2);
break;
}
}
System.out.println(succeed?"Nice":"Fucking fucked");
}
}