SmallSum-归并排序-小和问题（逆序对）

import java.util.Arrays;

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 (R == L) {
			return 0;
		}
		int mid = L + ((R - L) >> 1);
		return mergeSort(arr, L, mid) + mergeSort(arr, mid + 1, R) + merge(arr, L, mid, R);

	}

	public static int merge(int arr[], int L, int mid, int R) {
		int help[] = new int[R - L + 1];
		int i = 0;
		int p1 = L;
		int p2 = mid + 1;
		int res = 0;
		while (p1 <= mid && p2 <= R) {
			// 两部分已经排好序，p2后面的数值一样大于p1此时的值
			res += arr[p1] < arr[p2] ? (R - p2 + 1) * arr[p1] : 0;
			help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
		}
		while (p1 <= mid) {
			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;
	}

	// 比较函数
	public static int comparator(int arr[]) {
		if (arr == null || arr.length < 2) {
			return 0;
		}
		int res = 0;
		for (int i = 0; i < arr.length; i++) {
			for (int j = 0; j < i; j++) {
				res += arr[j] < arr[i] ? arr[j] : 0;
			}
		}
		return res;
	}

	// 获取随机数组
	public static int[] getRandomArray(int maxSize, int maxValue) {
		int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
		for (int i = 0; i < arr.length; i++) {
			// 获取 -maxVlaue + 1 ~ maxValue 的值
			arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
		}

		return arr;

	}

	// 复制数组
	public static int[] copyArray(int[] arr) {
		if (arr == null) {
			return null;
		}
		int[] book = new int[arr.length];
		for (int i = 0; i < arr.length; i++) {
			book[i] = arr[i];
		}
		return book;
	}

	// 比较
	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;

	}

	// 打印数组
	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) {
		// 测试次数
		int test = 50000;

		// 数组长度
		int maxSize = 100;

		// 最大数值
		int maxValue = 100;
		boolean flag = true;
		for (int i = 0; i < test; i++) {
			int arr1[] = getRandomArray(maxSize, maxValue);
			// 拷贝比较
			int arr2[] = copyArray(arr1);
			if (smallSum(arr1) != comparator(arr2)) {
				flag = false;
				break;
			}

		}
		System.out.println(flag ? "测试正常" : "发生错误");

		// 随机测试一组数据
		int arr[] = { 4, 2, 5, 1, 7, 3, 6 };
		// getRandomArray(maxSize, maxValue);

		System.out.println(Arrays.toString(arr));
		int t = smallSum(arr);
		System.out.println(t);

	}

}