数据结构与算法-归并排序

1.递归的概念

1.首先写一个小方法，对2个已经排好顺序的数组进行归并排序

代码如下：

 1  public static void MergeSortPart1()
 2         {
 3             int[] arr = { 1, 4, 7,8, 3, 6, 9 };
 4             var tempArr = new int[arr.Length];
 5             var tempIndex = 0;
 6 
 7             int leftIndex = 0;
 8 
 9             int mid = 3;
10 
11             int rightIndex = mid + 1;
12 
13             while (leftIndex <= mid && rightIndex< arr.Length)
14             {
15                 if (arr[leftIndex] <= arr[rightIndex])
16                 {
17                     tempArr[tempIndex] = arr[leftIndex];
18                     leftIndex++;
19                     tempIndex++;
20                 }
21                 else
22                 {
23                     tempArr[tempIndex] = arr[rightIndex];
24                     rightIndex++;
25                     tempIndex++;
26                 }
27             }
28 
29             if (leftIndex<=mid)
30             {
31                 while (leftIndex <= mid)
32                 {
33                     tempArr[tempIndex] = arr[leftIndex];
34                     tempIndex++;
35                     leftIndex++;
36                 }
37             }
38 
39             if (rightIndex<arr.Length)
40             {
41                 while (rightIndex <arr.Length)
42                 {
43                     tempArr[tempIndex] = arr[rightIndex];
44                     rightIndex++;
45                     tempIndex++;
46 
47                 }
48             }
49 
50             //PrintArr(tempArr);
51 
52 
53         }

2.改进上面的方法，参数是传入的数组，leftB的位置，mid的位置，rightB 的位置，就是对一个传入数组的指定边界的元素序列进行排序

 1 public static void MergeSortPart2(int[] arr, int leftB, int minPtr, int rightB)
 2         {
 3 
 4             var tempArr = new int[rightB - leftB + 1];
 5             var tempIndex = 0;
 6 
 7             int leftIndex = leftB;
 8 
 9             int mid = minPtr;
10 
11             int rightIndex = mid + 1;
12 
13             while (leftIndex <= mid && rightIndex <= rightB)
14             {
15                 if (arr[leftIndex] <= arr[rightIndex])
16                 {
17                     tempArr[tempIndex] = arr[leftIndex];
18                     leftIndex++;
19                     tempIndex++;
20                 }
21                 else
22                 {
23                     tempArr[tempIndex] = arr[rightIndex];
24                     rightIndex++;
25                     tempIndex++;
26                 }
27             }
28 
29             if (leftIndex <= mid)
30             {
31                 while (leftIndex <= mid)
32                 {
33                     tempArr[tempIndex] = arr[leftIndex];
34                     tempIndex++;
35                     leftIndex++;
36                 }
37             }
38 
39             if (rightIndex <= rightB)
40             {
41                 while (rightIndex <= rightB)
42                 {
43                     tempArr[tempIndex] = arr[rightIndex];
44                     rightIndex++;
45                     tempIndex++;
46 
47                 }
48             }
49 
50             //更新原数组元素
51             for (int i = 0; i < tempArr.Length; i++)
52             {
53                 arr[leftB+i] = tempArr[i];
54             }
55 
56             //Console.WriteLine($"当前排序：leftB:{leftB} ,rightB:{rightB},min:{minPtr}");
57 
58             //PrintArr(tempArr);
59         }

3.编写递归方法

 1 public static void MergeSortPart3(int[] arr ,int leftB,int rightB)
 2         {
 3             
 4             if (leftB==rightB)
 5             {
 6                 return;
 7             }
 8 
 9             //分成2半
10             var mid = (leftB + rightB) / 2;
11 
12             //左边排序
13             MergeSortPart3(arr, leftB, mid);
14 
15             //右边排序
16             MergeSortPart3(arr, mid+1 , rightB);
17 
18             //merge排序
19             MergeSortPart2(arr, leftB, mid, rightB);
20 
21         }

4.调用

1 public static void MergeSort(int[] arr)
2         {
3             MergeSortPart3(arr, 0, arr.Length-1);
4             
5         }

posted on 2020-10-14 22:13 雯烈阅读(140) 评论(0) 编辑收藏举报