算法设计与分析-4.1、归并排序

java实现的，递归的就写了这么长时间，非递归的就更不好想了，不过硬着头皮也要写出来

不加注释了，理解的基础上记住，反复看

看递归的方法看了半天，找了n个方法看，终于看懂了一个有详细解释的，通过写非递归的方法让我认识到了几点：

一、非递归太难了！！看不出来就一步一步的画图，先选择简单的输入，在选择其他输入
二、看同样一种算法，看会其中一种就行的，别看看这个又看看那个的，今天就耽误了一天的时间才看懂，一定要深入一个看懂，注意力集中

三、先看懂伪代码，然后转化成自己的

递归和非递归都用到了merge方法，就是合并是关键

递归：

public class MegerSort {private static void merge(int b[], int low, int mid, int high, int length) {int k = 0;int[] tmp = new int[length];int l_begin = low;int l_end = mid;int h_begin = mid + 1;int h_end = high;while (l_begin <= l_end && h_begin <= h_end) {if (b[l_begin] < b[h_begin]) {tmp[k++] = b[l_begin++];} else {tmp[k++] = b[h_begin++];}}while (l_begin <= l_end)tmp[k++] = b[l_begin++];while (h_begin <= h_end)tmp[k++] = b[h_begin++];for (int i = 0; i < (high - low + 1); i++)b[low + i] = tmp[i];            //要注意数组b的下标，low+i}private static void sort(int a[], int low, int high) {int mid = 0;int length = a.length;if (low < high) {mid = (low + high) / 2;sort(a, low, mid);sort(a, mid + 1, high);merge(a, low, mid, high, length);}for (int i = low; i <= high; i++)System.out.print(a[i] + "  ");System.out.println();}public static void main(String[] args) {int[] a = new int[] { 5, 8, 6, 2, 1, 4, 3, 10, 9, 7 };for (int i = 0; i < a.length; i++)System.out.print(a[i] + "  ");System.out.println();sort(a, 0, a.length-1);for (int i = 0; i < a.length; i++)System.out.print(a[i] + "  ");}}

非递归方法：

import java.util.Arrays;public class MegerSort {private static void merge(int[] x, int[] y, int low, int mid, int high) {int l_begin = low;int l_end = mid;int h_begin = mid + 1;int h_end = high;int k = low;while ((l_begin <= l_end) && (h_begin <= h_end)) {if (x[l_begin] < x[h_begin])y[k++] = x[l_begin++];elsey[k++] = x[h_begin++];}while (l_begin <= l_end)y[k++] = x[l_begin++];while (h_begin <= h_end)y[k++] = x[h_begin++];}private static void mergeSort(int[] a, int[] b, int s) {int i = 0;int length = a.length;while (i + 2 * s <= length) {merge(a, b, i, i + s - 1, i + 2 * s - 1);i += 2 * s;}if (i + s < length) {merge(a, b, i, i + s - 1, length - 1);} else {while (i < length) {b[i] = a[i];i++;}}}private static void sort(int[] a ){int[] b=new int[a.length];int s=1;while(s<a.length){mergeSort(a,b,s);s+=s;mergeSort(b,a,s);s+=s;}}public static void main(String[] args){int[] a={5, 8, 6, 2, 1, 1, 1, 11, 9, 7};System.out.println(Arrays.toString(a));sort(a);System.out.println(Arrays.toString(a));}}