97 lines
2.3 KiB
Java
97 lines
2.3 KiB
Java
package sorts;
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/**
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* Created by wangzheng on 2018/10/16.
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*/
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public class MergeSort {
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// 归并排序算法, a是数组，n表示数组大小
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public static void mergeSort(int[] a, int n) {
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mergeSortInternally(a, 0, n-1);
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}
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// 递归调用函数
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private static void mergeSortInternally(int[] a, int p, int r) {
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// 递归终止条件
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if (p >= r) return;
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// 取p到r之间的中间位置q,防止（p+r）的和超过int类型最大值
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int q = p + (r - p)/2;
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// 分治递归
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mergeSortInternally(a, p, q);
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mergeSortInternally(a, q+1, r);
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// 将A[p...q]和A[q+1...r]合并为A[p...r]
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merge(a, p, q, r);
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}
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private static void merge(int[] a, int p, int q, int r) {
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int i = p;
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int j = q+1;
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int k = 0; // 初始化变量i, j, k
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int[] tmp = new int[r-p+1]; // 申请一个大小跟a[p...r]一样的临时数组
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while (i<=q && j<=r) {
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if (a[i] <= a[j]) {
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tmp[k++] = a[i++]; // i++等于i:=i+1
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} else {
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tmp[k++] = a[j++];
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}
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}
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// 判断哪个子数组中有剩余的数据
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int start = i;
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int end = q;
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if (j <= r) {
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start = j;
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end = r;
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}
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// 将剩余的数据拷贝到临时数组tmp
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while (start <= end) {
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tmp[k++] = a[start++];
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}
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// 将tmp中的数组拷贝回a[p...r]
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for (i = 0; i <= r-p; ++i) {
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a[p+i] = tmp[i];
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}
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}
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/**
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* 合并(哨兵)
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*
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* @param arr
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* @param p
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* @param q
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* @param r
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*/
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private static void mergeBySentry(int[] arr, int p, int q, int r) {
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int[] leftArr = new int[q - p + 2];
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int[] rightArr = new int[r - q + 1];
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for (int i = 0; i <= q - p; i++) {
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leftArr[i] = arr[p + i];
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}
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// 第一个数组添加哨兵（最大值）
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leftArr[q - p + 1] = Integer.MAX_VALUE;
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for (int i = 0; i < r - q; i++) {
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rightArr[i] = arr[q + 1 + i];
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}
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// 第二个数组添加哨兵（最大值）
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rightArr[r-q] = Integer.MAX_VALUE;
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int i = 0;
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int j = 0;
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int k = p;
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while (k <= r) {
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// 当左边数组到达哨兵值时，i不再增加，直到右边数组读取完剩余值，同理右边数组也一样
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if (leftArr[i] <= rightArr[j]) {
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arr[k++] = leftArr[i++];
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} else {
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arr[k++] = rightArr[j++];
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}
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}
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}
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}
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