SmallSum.java

package Basics;

import java.util.Random;

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);   //求平均数防止溢出，并用移位运算符提高运算速度

        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 res = 0;
        int i = 0;
        int p1 = l;
        int p2 = m + 1;
        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];    //注意这里arr数组是l + i
        }
        return res;
    }


    //for test
    public static int comparator(int[] arr){
        if (arr == null || arr.length < 2){
            return 0;
        }
        int res = 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;
    }
    //for test
    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;
    }
    //for test
    public static int[] copyArray(int[] arr){
        if (arr == null){
            return null;
        }
        int[] res = new int[arr.length];
        for (int i = 0;i < res.length;i++){
            res[i] = arr[i];
        }
        return res;
    }
    //for test
    public static boolean isEqual(int[] arr1,int[] arr2){
        if ((arr1 == null && arr2 != null) || (arr1 != null && arr2 == null)){
            return false;
        }
        if (arr1 == null && arr2 == null){
            return true;
        }
        if (arr1.length != arr2.length){
            return false;
        }
        for (int i = 0;i < arr1.length;i++){
            if (arr1[i] != arr2[i]){
                return false;
            }
        }
        return true;
    }
    public static void printArray(int[] arr){
        if (arr == null){
            return;
        }
        for (int i = 0;i < arr.length;i++){
            System.out.println(arr[i] + "");
        }
        System.out.println();
    }
    // for test
    public static void main(String[] args) {
        int testTime = 500000;
        int maxSize = 100;
        int maxValue = 100;
        boolean succeed = true;
        for (int i = 0; i < testTime;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!");
    }
}