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package class04;
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public class Code03_ReversePair {
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public static int reverPairNumber(int[] arr) {
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if (arr == null || arr.length < 2) {
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return 0;
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}
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return process(arr, 0, arr.length - 1);
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}
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// arr[L..R]既要排好序，也要求逆序对数量返回
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// 所有merge时，产生的逆序对数量，累加，返回
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// 左 排序 merge并产生逆序对数量
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// 右 排序 merge并产生逆序对数量
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public static int process(int[] arr, int l, int r) {
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if (l == r) {
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return 0;
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}
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// l < r
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int mid = l + ((r - l) >> 1);
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return process(arr, l, mid) + process(arr, mid + 1, r) + merge(arr, l, mid, r);
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}
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public static int merge(int[] arr, int L, int m, int r) {
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int[] help = new int[r - L + 1];
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int i = help.length - 1;
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int p1 = m;
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int p2 = r;
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int res = 0;
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while (p1 >= L && p2 > m) {
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res += arr[p1] > arr[p2] ? (p2 - m) : 0;
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help[i--] = arr[p1] > arr[p2] ? arr[p1--] : arr[p2--];
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}
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while (p1 >= L) {
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help[i--] = arr[p1--];
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}
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while (p2 > m) {
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help[i--] = arr[p2--];
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}
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for (i = 0; i < help.length; i++) {
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arr[L + i] = help[i];
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}
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return res;
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}
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// for test
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public static int comparator(int[] arr) {
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int ans = 0;
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for (int i = 0; i < arr.length; i++) {
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for (int j = i + 1; j < arr.length; j++) {
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if (arr[i] > arr[j]) {
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ans++;
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}
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}
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}
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return ans;
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}
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// for test
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public static int[] generateRandomArray(int maxSize, int maxValue) {
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int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
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for (int i = 0; i < arr.length; i++) {
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arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
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}
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return arr;
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}
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// for test
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public static int[] copyArray(int[] arr) {
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if (arr == null) {
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return null;
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}
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int[] res = new int[arr.length];
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for (int i = 0; i < arr.length; i++) {
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res[i] = arr[i];
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}
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return res;
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}
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// for test
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public static boolean isEqual(int[] arr1, int[] arr2) {
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if ((arr1 == null && arr2 != null) || (arr1 != null && arr2 == null)) {
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return false;
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}
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if (arr1 == null && arr2 == null) {
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return true;
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}
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if (arr1.length != arr2.length) {
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return false;
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}
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for (int i = 0; i < arr1.length; i++) {
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if (arr1[i] != arr2[i]) {
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return false;
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}
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}
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return true;
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}
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// for test
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public static void printArray(int[] arr) {
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if (arr == null) {
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return;
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}
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for (int i = 0; i < arr.length; i++) {
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System.out.print(arr[i] + " ");
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}
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System.out.println();
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}
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// for test
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public static void main(String[] args) {
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int testTime = 500000;
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int maxSize = 100;
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int maxValue = 100;
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System.out.println("测试开始");
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for (int i = 0; i < testTime; i++) {
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int[] arr1 = generateRandomArray(maxSize, maxValue);
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int[] arr2 = copyArray(arr1);
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if (reverPairNumber(arr1) != comparator(arr2)) {
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System.out.println("Oops!");
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printArray(arr1);
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printArray(arr2);
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break;
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}
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}
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System.out.println("测试结束");
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}
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}
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