归并排序

归并排序分两步：

1、分：二分法，让每个组元素都有序，只有每个组只有一个元素时才一定有序。

2、合：按照二分法分出的小组组合。

void Merge(int arr[], int tmp[], int startIndex, int midIndex, int endIndex){int i = startIndex;int j = midIndex + 1;int k = startIndex;while (i != midIndex + 1 && j != endIndex + 1){if (arr[i] > arr[j]){tmp[k++] = arr[j++];}else{tmp[k++] = arr[i++];}}while (i != midIndex + 1){tmp[k++] = arr[i++];}while (j != endIndex + 1){tmp[k++] = arr[j++];}for (int i = startIndex; i <= endIndex; ++i){arr[i] = tmp[i];}}void MergeSort(int arr[], int tmp[], int startIndex, int endIndex){if (startIndex < endIndex){int midIndex = (startIndex + endIndex) / 2;MergeSort(arr, tmp, startIndex, midIndex);MergeSort(arr, tmp, midIndex + 1, endIndex);Merge(arr, tmp, startIndex, midIndex, endIndex);}}