归并排序

归并排序

归并排序，基本思路就是将数组分成二组A，B，如果这二组组内的数据都是有序的，那么就可以很方便的将这二组数据进行排序。为了让这二组数据有序，可以将A，B组各自再分成二组，依次类推，当分出来的小组只有一个数据时，可以认为这个小组组内已经达到了有序，然后再合并相邻的二个小组就可以了。这样通过先递归的分解数列，再合并数列就完成了归并排序。

归并排序有两种实现方法：自底向上和自顶向下

自底向上的方法，效率高，但是可读性较差

void mergeSort_nonRecursively(int data[], int length){if (data == NULL || length <= 1)return;int *result = (int *)malloc(sizeof(int) * length);if (result == NULL)return;int left, right;int left_index, right_index;int step;for (step = 1; step < length; step *= 2){left = 0;right = left + step;left_index = right_index = 0;int i = 0;while ((right + right_index) < length){while (left_index < step && right_index < step && ((right + right_index) < length)){if (data[left + left_index] < data[right + right_index]){result[i ++] = data[left + left_index];++ left_index;}else{result[i ++] = data[right + right_index];++ right_index;}}while (left_index < step){result[i ++] = data[left + left_index];++ left_index;}while (right_index < step && ((right + right_index) < length)){result[i ++] = data[right + right_index];++ right_index;}left += step * 2;right += step * 2;left_index = 0;right_index = 0;}memcpy(data, result, i * sizeof(int));}free(result);}

自顶向下的方法，其实也就是递归的方法，可读性较好，但效率比非递归的要差些

void mergeSort(int data[], int first, int last, int result[]){if (data == NULL || first < 0 || first >= last || result == NULL)return;int mid = (first + last) / 2;mergeSort(data, first, mid, result);mergeSort(data, mid + 1, last, result);int left_index = first;int right_index = mid + 1;int res_index = 0;while (left_index <= mid && right_index <= last){if (data[left_index] > data[right_index]) //<升序，>降序result[res_index ++] = data[left_index ++];elseresult[res_index ++] = data[right_index ++];}while (left_index <= mid)result[res_index ++] = data[left_index ++];while (right_index <= last)result[res_index ++] = data[right_index ++];int i = 0;for (; i < res_index; i++)data[first + i] = result[i];}