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package class04;
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public class Code02_SmallSum {
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public static int smallSum(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，也要求小和返回
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// 所有merge时，产生的小和，累加
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// 左 排序 merge
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// 右 排序 merge
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// merge
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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
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process(arr, l, mid)
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+
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process(arr, mid + 1, r)
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+
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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 = 0;
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int p1 = L;
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int p2 = m + 1;
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int res = 0;
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while (p1 <= m && p2 <= r) {
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res += arr[p1] < arr[p2] ? (r - p2 + 1) * arr[p1] : 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 <= m) {
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help[i++] = arr[p1++];
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}
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while (p2 <= r) {
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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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if (arr == null || arr.length < 2) {
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return 0;
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}
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int res = 0;
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for (int i = 1; i < arr.length; i++) {
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for (int j = 0; j < i; j++) {
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res += arr[j] < arr[i] ? arr[j] : 0;
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}
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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[] 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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boolean succeed = true;
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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 (smallSum(arr1) != comparator(arr2)) {
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succeed = false;
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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(succeed ? "Nice!" : "Fucking fucked!");
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}
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}
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