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本帖最后由 zfzhuman123 于 2011-7-28 11:24 编辑
实用排序算法(复杂度小于等于O(n^2))中效率最低但实现并不是最简单的的两个,C、C++教材却总喜欢拿来大讲特讲,非常不利于初学者养成“程序效率”的思维。
实际上,各种排序算法里,除了堆排序实现较为复杂外,从代码量的角度,大多数算法都不比冒泡、选择算法复杂多少。
举几个高速的排序算法的例子,这些才适合进入教材
鸽巢排序,排序字节串、宽字节串最快的排序算法,计数排序的变种(将计数缓冲区大小固定,少一次遍历开销),速度是STL中std::sort的20多倍,更重要的是实现极其简单!缺点是需要一个size至少等于待排序数组取值范围的缓冲区,不适合int等大范围数据
C/C++ code- void PigeonholeSort(BYTE *array, int length)
- {
- int b[256] = {0};
- int i,k,j = 0;
- for(i=0; i<length; i++)
- b[array[i]]++;
- for(i=0; i<256; i++)
- for(k=0; k<b[i]; k++)
- array[j++] = i;
- }
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多一次遍历的计数排序,排序字节串的话速度约是鸽巢排序的一半
C/C++ code- void CountingSort(BYTE *array, int length)
- {
- int t;
- int i, z = 0;
- BYTE min,max;
- int *count;
- min = max = array[0];
- for(i=0; i<length; i++)
- {
- if(array[i] < min)
- min = array[i];
- else if(array[i] > max)
- max = array[i];
- }
- count = (int*)malloc((max-min+1)*sizeof(int));
- for(i=0; i<max-min+1; i++)
- count[i] = 0;
- for(i = 0; i < length; i++)
- count[array[i]-min]++;
- for(t = 0; t <= 255; t++)
- for(i = 0; i < count[t-min]; i++)
- array[z++] = (BYTE)t;
- free(count);
- }
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快速排序,快排最标准的递归实现,速度约是std::sort的一半
C/C++ code
- void swap(BYTE *a,BYTE *b)
- {
- BYTE tmp;
- if ( a != b )
- {
- tmp = *a;
- *a = *b;
- *b = tmp;
- }
- }
- int partition(BYTE *arr,int left, int right)
- {
- int i = left - 1, j = right;
- BYTE v = arr[right];
- while(1)
- {
- while(arr[++i] < v);
- while(arr[--j] > v)
- if(j == 1)
- break;
- if(i >= j)
- break;
- swap(&arr[i],&arr[j]);
- }
- swap(&arr[i],&arr[right]);
- return i;
- }
- void quicksort(BYTE *arr, int left, int right)
- {
- if (left < right)
- {
- int i = partition(arr,left,right);
- quicksort(arr,left,i-1);
- quicksort(arr,i+1,right);
- }
- }
- void QuickSort(BYTE *array,int length)
- {
- quicksort(array,0,length-1);
- }
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这是速度与std::sort相当的三路划分快排
C/C++ code- void swap(BYTE *a,BYTE *b)
- {
- BYTE tmp;
- if ( a != b )
- {
- tmp = *a;
- *a = *b;
- *b = tmp;
- }
- }
- void quicksort(BYTE *arr, int left, int right)
- {
- if (left < right)
- {
- BYTE v = arr[right];
- int i = left - 1,j = right,p = left - 1,q = right,k=0;
- while (1)
- {
- while (arr[++i] < v);
- while (arr[--j] > v)
- if(j==left)
- break;
- if (i >= j)
- break;
- swap(&arr[i], &arr[j]);
- if(arr[i] == v)
- {
- p++;
- swap(&arr[p],&arr[i]);
- }
- if(arr[j] == v)
- {
- q--;
- swap(&arr[q],&arr[j]);
- }
- }
- swap(&arr[i],&arr[right]);
- j = i - 1;
- i++;
- for(k=left; k<=p; k++,j--)
- swap(&arr[k],&arr[j]);
- for(k=right-1; k>=q; k--,i++)
- swap(&arr[k],&arr[i]);
- quicksort(arr,left,j);
- quicksort(arr,i,right);
- }
- }
- void QuickSort(BYTE *array,int length)
- {
- quicksort(array,0,length-1);
- }
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相当简单的梳排序,效率是std::sort的三分之一
C/C++ code
- void CombSort(BYTE *arr, int size)
- {
- UINT gap = size, swapped = 1, i = 0;
- BYTE swap = 0;
- while ((gap > 1) || swapped)
- {
- if (gap > 1)
- gap = gap / 1.3;
- swapped = 0;
- i = 0;
- while ((gap + i) < size)
- {
- if (arr[i] - arr[i + gap] > 0)
- {
- swap = arr[i];
- arr[i] = arr[i + gap];
- arr[i + gap] = swap;
- swapped = 1;
- }
- ++i;
- }
- }
- }
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LSD基数排序,与std::sort速度相当,但是需要一个与输入缓冲一样大的缓冲区
C/C++ code
- #define R 256
- #define digit(a, d) ( a >> 8*d )
- static BYTE *aux;
- void radix_sort(BYTE *arr, int left, int right)
- {
- if(left < right)
- {
- int d = 0;
- for(d=3; d>=0; d--)
- {
- int i=0, j=0, count[R+1];
- for(j=0; j<R; j++)
- count[j] = 0;
- for(i=left; i<=right; i++)
- count[digit(arr[i],d) + 1]++;
- for(j=1; j<R; j++)
- count[j] += count[j-1];
- for(i=left; i<=right; i++)
- aux[count[digit(arr[i],d)]++] = arr[i];
- for(i=left; i<=right; i++)
- arr[i] = aux[i-1];
- }
- }
- }
- void RadixSort(BYTE *array,int length)
- {
- aux = (BYTE*)malloc(length);
- radix_sort(array,0,length-1);
- free(aux);
- }
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归并排序,效率越是std::sort的六分之一,通常的实现是递归,但和快排不同,归并改循环极其容易
C/C++ code- void merge(BYTE *array, int low, int mid, int high)
- {
- int i, k;
- BYTE *temp = (BYTE *) malloc(high-low+1);
- int begin1 = low;
- int end1 = mid;
- int begin2 = mid + 1;
- int end2 = high;
- for (k = 0; begin1 <= end1 && begin2 <= end2; ++k)
- if(array[begin1]<array[begin2])
- temp[k] = array[begin1++];
- else
- temp[k] = array[begin2++];
- while(begin1<=end1)
- temp[k++] = array[begin1++];
- while(begin2<=end2)
- temp[k++] = array[begin2++];
- for (i = 0; i < (high-low+1); i++)
- array[low+i] = temp[i];
- free(temp);
- }
- void merge_sort(BYTE *array, UINT first, UINT last)
- {
- UINT mid,i;
- for(mid=1; mid<=last-first; mid += mid)
- for(i=first; i<=last-mid; i+=mid+mid)
- merge(array,i,i+mid-1,min(i+mid+mid-1,last));
- }
- void MergeSort(BYTE *array, UINT length)
- {
- merge_sort(array,0,length-1);
- }
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