求数组小和(递归求解)

import java.util.Arrays;

/**
 * 求数组小和
 * 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，叫数组小和。
 * 归并排序(递归)
 */
public class SmallSum {

    public static void main(String[] args) {
        // 测试次数
        int times = 50000;

        int maxNum = 100;
        int maxSize = 100;
        for (int i = 0; i < times; i++) {
            // 生成随机数组
            int[] arr1 = generateArray(maxNum, maxSize);
            int[] arr2 = copyArray(arr1);
            // 小和
            int sum1 = smallSum(arr1);
            int sum2 = smallSum2(arr2);
            // 比较结果
            if (sum1 != sum2) {
                System.out.println("the result is error at " + i);
                System.out.println(sum1);
                System.out.println(sum2);
                System.out.println(Arrays.toString(arr1));
                System.out.println(Arrays.toString(arr2));
                return;
            }
        }
        System.out.println("the result is right");
    }

    public static int smallSum(int[] arr) {
        if (arr == null || arr.length < 2) {
            return 0;
        }
        return smallSum(arr, 0, arr.length - 1);
    }

    public static int smallSum(int[] arr, int left, int right) {
        if (left == right) {
            return 0;
        }
        int mid = left + ((right - left) >> 1);
        return smallSum(arr, left, mid)
                + smallSum(arr, mid + 1, right)
                + merge(arr, left, mid, right);
    }

    public static int merge(int[] arr, int left, int mid, int right) {
        int[] help = new int[right - left + 1];
        int res = 0;
        int index = 0;
        int po1 = left;
        int po2 = mid + 1;
        while (po1 <= mid && po2 <= right) {
            res += arr[po1] < arr[po2] ? (right - po2 + 1) * arr[po1] : 0;
            help[index++] = arr[po1] < arr[po2] ? arr[po1++] : arr[po2++];
        }
        while (po1 <= mid) {
            help[index++] = arr[po1++];
        }
        while (po2 <= right) {
            help[index++] = arr[po2++];
        }
        for (int i = 0; i < help.length; i++) {
            arr[left + i] = help[i];
        }
        return res;
    }

    /**
     * 生成随机数组
     *
     * @param maxNum  最大数
     * @param maxSize 数组最大大小
     * @return 随机数组
     */
    private static int[] generateArray(int maxNum, int maxSize) {
        int[] arr = new int[(int) (maxSize * Math.random())];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = (int) (maxNum * Math.random());
        }
        return arr;
    }

    /**
     * 遍历求出数组小和
     *
     * @param arr 数组
     * @return 小和
     */
    public static int smallSum2(int[] arr) {
        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;
    }

    /**
     * 复制数组
     *
     * @param arr 要复制的数组
     * @return 复制的数组
     */
    private static int[] copyArray(int[] arr) {
        if (arr == null) {
            return null;
        }
        int[] copy = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            copy[i] = arr[i];
        }
        return copy;
    }

}

/* 如有错误，欢迎批评指正 */