算法（一）：排序

package com.sort;

//不稳定
public class 快速排序 {
    public static void main(String[] args) {
        int[] a={49,38,65,97,76,13,27,49,78,34,12,64,1,8};
        System.out.println("排序之前：");
        for (int i = 0; i < a.length; i++) {
            System.out.print(a[i]+" ");
        }
        //快速排序
        quick(a);
        System.out.println();
        System.out.println("排序之后：");
        for (int i = 0; i < a.length; i++) {
            System.out.print(a[i]+" ");
        }
    }

    private static void quick(int[] a) {
        if(a.length>0){
            quickSort(a,0,a.length-1);
        }
    }

    private static void quickSort(int[] a, int low, int high) {
        if(low<high){ //如果不加这个判断递归会无法退出导致堆栈溢出异常
            int middle = getMiddle(a,low,high);
            quickSort(a, 0, middle-1);
            quickSort(a, middle+1, high);
        }
    }

    private static int getMiddle(int[] a, int low, int high) {
        int temp = a[low];//基准元素
        while(low<high){
            //找到比基准元素小的元素位置
            while(low<high && a[high]>=temp){
                high--;
            }
            a[low] = a[high]; 
            while(low<high && a[low]<=temp){
                low++;
            }
            a[high] = a[low];
        }
        a[low] = temp;
        return low;
    }
}

 1 package com.sort;
 2 
 3 //稳定
 4 public class 归并排序 {
 5     public static void main(String[] args) {
 6         int[] a={49,38,65,97,76,13,27,49,78,34,12,64,1,8};
 7         System.out.println("排序之前：");
 8         for (int i = 0; i < a.length; i++) {
 9             System.out.print(a[i]+" ");
10         }
11         //归并排序
12         mergeSort(a,0,a.length-1);
13         System.out.println();
14         System.out.println("排序之后：");
15         for (int i = 0; i < a.length; i++) {
16             System.out.print(a[i]+" ");
17         }
18     }
19 
20     private static void mergeSort(int[] a, int left, int right) {
21         if(left<right){
22             int middle = (left+right)/2;
23             //对左边进行递归
24             mergeSort(a, left, middle);
25             //对右边进行递归
26             mergeSort(a, middle+1, right);
27             //合并
28             merge(a,left,middle,right);
29         }
30     }
31 
32     private static void merge(int[] a, int left, int middle, int right) {
33         int[] tmpArr = new int[a.length];
34         int mid = middle+1; //右边的起始位置
35         int tmp = left;
36         int third = left;
37         while(left<=middle && mid<=right){
38             //从两个数组中选取较小的数放入中间数组
39             if(a[left]<=a[mid]){
40                 tmpArr[third++] = a[left++];
41             }else{
42                 tmpArr[third++] = a[mid++];
43             }
44         }
45         //将剩余的部分放入中间数组
46         while(left<=middle){
47             tmpArr[third++] = a[left++];
48         }
49         while(mid<=right){
50             tmpArr[third++] = a[mid++];
51         }
52         //将中间数组复制回原数组
53         while(tmp<=right){
54             a[tmp] = tmpArr[tmp++];
55         }
56     }
57 }

 1 package com.sort;
 2 
 3 import java.util.ArrayList;
 4 import java.util.List;
 5 //稳定
 6 public class 基数排序 {
 7     public static void main(String[] args) {
 8         int[] a={49,38,65,97,176,213,227,49,78,34,12,164,11,18,1};
 9         System.out.println("排序之前：");
10         for (int i = 0; i < a.length; i++) {
11             System.out.print(a[i]+" ");
12         }
13         //基数排序
14         sort(a);
15         System.out.println();
16         System.out.println("排序之后：");
17         for (int i = 0; i < a.length; i++) {
18             System.out.print(a[i]+" ");
19         }
20     }
21 
22     private static void sort(int[] array) {
23         //找到最大数，确定要排序几趟
24         int max = 0;
25         for (int i = 0; i < array.length; i++) {
26             if(max<array[i]){
27                 max = array[i];
28             }
29         }
30         //判断位数
31         int times = 0;
32         while(max>0){
33             max = max/10;
34             times++;
35         }
36         //建立十个队列
37         List<ArrayList> queue = new ArrayList<ArrayList>();
38         for (int i = 0; i < 10; i++) {
39             ArrayList queue1 = new ArrayList();
40             queue.add(queue1);
41         }
42         //进行times次分配和收集
43         for (int i = 0; i < times; i++) {
44             //分配
45             for (int j = 0; j < array.length; j++) {
46                 int x = array[j]%(int)Math.pow(10, i+1)/(int)Math.pow(10, i);
47                 ArrayList queue2 = queue.get(x);
48                 queue2.add(array[j]);
49                 queue.set(x,queue2);
50             }
51             //收集
52             int count = 0;
53             for (int j = 0; j < 10; j++) {
54                 while(queue.get(j).size()>0){
55                     ArrayList<Integer> queue3 = queue.get(j);
56                     array[count] = queue3.get(0);
57                     queue3.remove(0);
58                     count++;
59                 }
60             }
61         }
62     }
63 }