merge sort

 1 public class Sort {
 2         public static void main(String[] args){
 3             int[] arr = new int[]{8,30,19,1,45,3,6};
 4             System.out.println("before sort: ");
 5             printarr(arr);
 6             mergesort(arr,0,arr.length-1);
 7             System.out.println("merge sort result: ");
 8             printarr(arr);
 9             System.out.println();
10         }
11     
12         //归并排序
13         //采用分治的思想，递归调用，分成两份，每份各自排序，然后再合并结果；合并时采用三个指针
14         private static  void mergesort(int[] arr,int start,int end) {
15             if(end>start){
16                 int divide = (start+end)/2;
17                 mergesort(arr,start,divide);
18                 mergesort(arr,divide+1,end);
19                 merge(arr,start,end,divide);
20             }
21         }
22         private static void merge(int[] arr,int start,int end,int divide) {
23             //三个指针的初始化
24             int i = 0;
25             int j = 0;
26             int k = start;
27             //创建左右子数组，并赋值
28             int lsize = divide-start+1;
29             int rsize = end-divide;
30             int[] arrL = new int[divide-start+1];
31             int[] arrR = new int[end-divide];
32             for(int n =0;n<lsize;n++){
33                 arrL[n] = arr[n+start];
34             }
35             for(int n =0;n<rsize;n++){
36                 arrR[n] = arr[n+divide+1];
37             }
38             //三个指针移动，进行合并
39             while(k<end+1){
40                 if(arrL[i]<arrR[j]){
41                     arr[k]=arrL[i];
42                     k++;
43                     i++;
44                     if(i>=lsize)
45                         break;
46                 }else{
47                     arr[k]=arrR[j];
48                     k++;
49                     j++;
50                     if(j>=rsize)
51                         break;
52                 }
53             }
54             //把剩余的都填进arr
55             while(i<lsize){
56                 arr[k]=arrL[i];
57                 k++;
58                 i++;
59             }
60             while(j<rsize){
61                 arr[k]=arrR[j];
62                 k++;
63                 j++;
64             }
65             System.out.print("    本次递归的结果：");
66             printarr(arr);
67         }
68         public static void printarr(int[] arr){
69             for(int i = 0;i<arr.length;i++){
70                 if(i!=arr.length-1){
71                     System.out.print(arr[i]+",");
72                 }else{
73                     System.out.print(arr[i]);
74                     System.out.println();
75                 }
76                     
77             }
78         }
79     }