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：

 public static void MergeSortPart1()
        {
            int[] arr = { 1, 4, 7,8, 3, 6, 9 };
            var tempArr = new int[arr.Length];
            var tempIndex = 0;

            int leftIndex = 0;

            int mid = 3;

            int rightIndex = mid + 1;

            while (leftIndex <= mid && rightIndex< arr.Length)
            {
                if (arr[leftIndex] <= arr[rightIndex])
                {
                    tempArr[tempIndex] = arr[leftIndex];
                    leftIndex++;
                    tempIndex++;
                }
                else
                {
                    tempArr[tempIndex] = arr[rightIndex];
                    rightIndex++;
                    tempIndex++;
                }
            }

            if (leftIndex<=mid)
            {
                while (leftIndex <= mid)
                {
                    tempArr[tempIndex] = arr[leftIndex];
                    tempIndex++;
                    leftIndex++;
                }
            }

            if (rightIndex<arr.Length)
            {
                while (rightIndex <arr.Length)
                {
                    tempArr[tempIndex] = arr[rightIndex];
                    rightIndex++;
                    tempIndex++;

                }
            }

            //PrintArr(tempArr);


        }

 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

public static void MergeSortPart2(int[] arr, int leftB, int minPtr, int rightB)
        {

            var tempArr = new int[rightB - leftB + 1];
            var tempIndex = 0;

            int leftIndex = leftB;

            int mid = minPtr;

            int rightIndex = mid + 1;

            while (leftIndex <= mid && rightIndex <= rightB)
            {
                if (arr[leftIndex] <= arr[rightIndex])
                {
                    tempArr[tempIndex] = arr[leftIndex];
                    leftIndex++;
                    tempIndex++;
                }
                else
                {
                    tempArr[tempIndex] = arr[rightIndex];
                    rightIndex++;
                    tempIndex++;
                }
            }

            if (leftIndex <= mid)
            {
                while (leftIndex <= mid)
                {
                    tempArr[tempIndex] = arr[leftIndex];
                    tempIndex++;
                    leftIndex++;
                }
            }

            if (rightIndex <= rightB)
            {
                while (rightIndex <= rightB)
                {
                    tempArr[tempIndex] = arr[rightIndex];
                    rightIndex++;
                    tempIndex++;

                }
            }

            //更新原数组元素
            for (int i = 0; i < tempArr.Length; i++)
            {
                arr[leftB+i] = tempArr[i];
            }

            //Console.WriteLine($"当前排序：leftB:{leftB} ,rightB:{rightB},min:{minPtr}");

            //PrintArr(tempArr);
        }

3.编写递归方法

public static void MergeSortPart3(int[] arr ,int leftB,int rightB)
        {
            
            if (leftB==rightB)
            {
                return;
            }

            //分成2半
            var mid = (leftB + rightB) / 2;

            //左边排序
            MergeSortPart3(arr, leftB, mid);

            //右边排序
            MergeSortPart3(arr, mid+1 , rightB);

            //merge排序
            MergeSortPart2(arr, leftB, mid, rightB);

        }

4.调用

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