我知道这个问题不太具体。
我想要的只是有人告诉我如何将普通的归并排序转换为就地归并排序(或具有常量额外空间开销的归并排序)。
我所能找到的(在网上)都是说“这太复杂”或“超出了本文的范围”的页面。
唯一已知的就地合并(没有任何额外空间)的方法太复杂，无法简化为实用程序。(从这里拍摄)
即使它太复杂，如何使归并排序的基本概念是什么?
当前回答
C语言中无缓冲区归并排序的一个例子。
#define SWAP(type, a, b) \
do { type t=(a);(a)=(b);(b)=t; } while (0)
static void reverse_(int* a, int* b)
{
for ( --b; a < b; a++, b-- )
SWAP(int, *a, *b);
}
static int* rotate_(int* a, int* b, int* c)
/* swap the sequence [a,b) with [b,c). */
{
if (a != b && b != c)
{
reverse_(a, b);
reverse_(b, c);
reverse_(a, c);
}
return a + (c - b);
}
static int* lower_bound_(int* a, int* b, const int key)
/* find first element not less than @p key in sorted sequence or end of
* sequence (@p b) if not found. */
{
int i;
for ( i = b-a; i != 0; i /= 2 )
{
int* mid = a + i/2;
if (*mid < key)
a = mid + 1, i--;
}
return a;
}
static int* upper_bound_(int* a, int* b, const int key)
/* find first element greater than @p key in sorted sequence or end of
* sequence (@p b) if not found. */
{
int i;
for ( i = b-a; i != 0; i /= 2 )
{
int* mid = a + i/2;
if (*mid <= key)
a = mid + 1, i--;
}
return a;
}
static void ip_merge_(int* a, int* b, int* c)
/* inplace merge. */
{
int n1 = b - a;
int n2 = c - b;
if (n1 == 0 || n2 == 0)
return;
if (n1 == 1 && n2 == 1)
{
if (*b < *a)
SWAP(int, *a, *b);
}
else
{
int* p, * q;
if (n1 <= n2)
p = upper_bound_(a, b, *(q = b+n2/2));
else
q = lower_bound_(b, c, *(p = a+n1/2));
b = rotate_(p, b, q);
ip_merge_(a, p, b);
ip_merge_(b, q, c);
}
}
void mergesort(int* v, int n)
{
if (n > 1)
{
int h = n/2;
mergesort(v, h); mergesort(v+h, n-h);
ip_merge_(v, v+h, v+n);
}
}
一个自适应归并排序的例子(优化)。
添加支持代码和修改，以在任何大小的辅助缓冲区可用时加速合并(在没有额外内存的情况下仍然可以工作)。使用前向和后向合并、环旋转、小序列合并和排序以及迭代合并排序。
#include <stdlib.h>
#include <string.h>
static int* copy_(const int* a, const int* b, int* out)
{
int count = b - a;
if (a != out)
memcpy(out, a, count*sizeof(int));
return out + count;
}
static int* copy_backward_(const int* a, const int* b, int* out)
{
int count = b - a;
if (b != out)
memmove(out - count, a, count*sizeof(int));
return out - count;
}
static int* merge_(const int* a1, const int* b1, const int* a2,
const int* b2, int* out)
{
while ( a1 != b1 && a2 != b2 )
*out++ = (*a1 <= *a2) ? *a1++ : *a2++;
return copy_(a2, b2, copy_(a1, b1, out));
}
static int* merge_backward_(const int* a1, const int* b1,
const int* a2, const int* b2, int* out)
{
while ( a1 != b1 && a2 != b2 )
*--out = (*(b1-1) > *(b2-1)) ? *--b1 : *--b2;
return copy_backward_(a1, b1, copy_backward_(a2, b2, out));
}
static unsigned int gcd_(unsigned int m, unsigned int n)
{
while ( n != 0 )
{
unsigned int t = m % n;
m = n;
n = t;
}
return m;
}
static void rotate_inner_(const int length, const int stride,
int* first, int* last)
{
int* p, * next = first, x = *first;
while ( 1 )
{
p = next;
if ((next += stride) >= last)
next -= length;
if (next == first)
break;
*p = *next;
}
*p = x;
}
static int* rotate_(int* a, int* b, int* c)
/* swap the sequence [a,b) with [b,c). */
{
if (a != b && b != c)
{
int n1 = c - a;
int n2 = b - a;
int* i = a;
int* j = a + gcd_(n1, n2);
for ( ; i != j; i++ )
rotate_inner_(n1, n2, i, c);
}
return a + (c - b);
}
static void ip_merge_small_(int* a, int* b, int* c)
/* inplace merge.
* @note faster for small sequences. */
{
while ( a != b && b != c )
if (*a <= *b)
a++;
else
{
int* p = b+1;
while ( p != c && *p < *a )
p++;
rotate_(a, b, p);
b = p;
}
}
static void ip_merge_(int* a, int* b, int* c, int* t, const int ts)
/* inplace merge.
* @note works with or without additional memory. */
{
int n1 = b - a;
int n2 = c - b;
if (n1 <= n2 && n1 <= ts)
{
merge_(t, copy_(a, b, t), b, c, a);
}
else if (n2 <= ts)
{
merge_backward_(a, b, t, copy_(b, c, t), c);
}
/* merge without buffer. */
else if (n1 + n2 < 48)
{
ip_merge_small_(a, b, c);
}
else
{
int* p, * q;
if (n1 <= n2)
p = upper_bound_(a, b, *(q = b+n2/2));
else
q = lower_bound_(b, c, *(p = a+n1/2));
b = rotate_(p, b, q);
ip_merge_(a, p, b, t, ts);
ip_merge_(b, q, c, t, ts);
}
}
static void ip_merge_chunk_(const int cs, int* a, int* b, int* t,
const int ts)
{
int* p = a + cs*2;
for ( ; p <= b; a = p, p += cs*2 )
ip_merge_(a, a+cs, p, t, ts);
if (a+cs < b)
ip_merge_(a, a+cs, b, t, ts);
}
static void smallsort_(int* a, int* b)
/* insertion sort.
* @note any stable sort with low setup cost will do. */
{
int* p, * q;
for ( p = a+1; p < b; p++ )
{
int x = *p;
for ( q = p; a < q && x < *(q-1); q-- )
*q = *(q-1);
*q = x;
}
}
static void smallsort_chunk_(const int cs, int* a, int* b)
{
int* p = a + cs;
for ( ; p <= b; a = p, p += cs )
smallsort_(a, p);
smallsort_(a, b);
}
static void mergesort_lower_(int* v, int n, int* t, const int ts)
{
int cs = 16;
smallsort_chunk_(cs, v, v+n);
for ( ; cs < n; cs *= 2 )
ip_merge_chunk_(cs, v, v+n, t, ts);
}
static void* get_buffer_(int size, int* final)
{
void* p = NULL;
while ( size != 0 && (p = malloc(size)) == NULL )
size /= 2;
*final = size;
return p;
}
void mergesort(int* v, int n)
{
/* @note buffer size may be in the range [0,(n+1)/2]. */
int request = (n+1)/2 * sizeof(int);
int actual;
int* t = (int*) get_buffer_(request, &actual);
/* @note allocation failure okay. */
int tsize = actual / sizeof(int);
mergesort_lower_(v, n, t, tsize);
free(t);
}
其他回答
C语言中无缓冲区归并排序的一个例子。
#define SWAP(type, a, b) \
do { type t=(a);(a)=(b);(b)=t; } while (0)
static void reverse_(int* a, int* b)
{
for ( --b; a < b; a++, b-- )
SWAP(int, *a, *b);
}
static int* rotate_(int* a, int* b, int* c)
/* swap the sequence [a,b) with [b,c). */
{
if (a != b && b != c)
{
reverse_(a, b);
reverse_(b, c);
reverse_(a, c);
}
return a + (c - b);
}
static int* lower_bound_(int* a, int* b, const int key)
/* find first element not less than @p key in sorted sequence or end of
* sequence (@p b) if not found. */
{
int i;
for ( i = b-a; i != 0; i /= 2 )
{
int* mid = a + i/2;
if (*mid < key)
a = mid + 1, i--;
}
return a;
}
static int* upper_bound_(int* a, int* b, const int key)
/* find first element greater than @p key in sorted sequence or end of
* sequence (@p b) if not found. */
{
int i;
for ( i = b-a; i != 0; i /= 2 )
{
int* mid = a + i/2;
if (*mid <= key)
a = mid + 1, i--;
}
return a;
}
static void ip_merge_(int* a, int* b, int* c)
/* inplace merge. */
{
int n1 = b - a;
int n2 = c - b;
if (n1 == 0 || n2 == 0)
return;
if (n1 == 1 && n2 == 1)
{
if (*b < *a)
SWAP(int, *a, *b);
}
else
{
int* p, * q;
if (n1 <= n2)
p = upper_bound_(a, b, *(q = b+n2/2));
else
q = lower_bound_(b, c, *(p = a+n1/2));
b = rotate_(p, b, q);
ip_merge_(a, p, b);
ip_merge_(b, q, c);
}
}
void mergesort(int* v, int n)
{
if (n > 1)
{
int h = n/2;
mergesort(v, h); mergesort(v+h, n-h);
ip_merge_(v, v+h, v+n);
}
}
一个自适应归并排序的例子(优化)。
添加支持代码和修改，以在任何大小的辅助缓冲区可用时加速合并(在没有额外内存的情况下仍然可以工作)。使用前向和后向合并、环旋转、小序列合并和排序以及迭代合并排序。
#include <stdlib.h>
#include <string.h>
static int* copy_(const int* a, const int* b, int* out)
{
int count = b - a;
if (a != out)
memcpy(out, a, count*sizeof(int));
return out + count;
}
static int* copy_backward_(const int* a, const int* b, int* out)
{
int count = b - a;
if (b != out)
memmove(out - count, a, count*sizeof(int));
return out - count;
}
static int* merge_(const int* a1, const int* b1, const int* a2,
const int* b2, int* out)
{
while ( a1 != b1 && a2 != b2 )
*out++ = (*a1 <= *a2) ? *a1++ : *a2++;
return copy_(a2, b2, copy_(a1, b1, out));
}
static int* merge_backward_(const int* a1, const int* b1,
const int* a2, const int* b2, int* out)
{
while ( a1 != b1 && a2 != b2 )
*--out = (*(b1-1) > *(b2-1)) ? *--b1 : *--b2;
return copy_backward_(a1, b1, copy_backward_(a2, b2, out));
}
static unsigned int gcd_(unsigned int m, unsigned int n)
{
while ( n != 0 )
{
unsigned int t = m % n;
m = n;
n = t;
}
return m;
}
static void rotate_inner_(const int length, const int stride,
int* first, int* last)
{
int* p, * next = first, x = *first;
while ( 1 )
{
p = next;
if ((next += stride) >= last)
next -= length;
if (next == first)
break;
*p = *next;
}
*p = x;
}
static int* rotate_(int* a, int* b, int* c)
/* swap the sequence [a,b) with [b,c). */
{
if (a != b && b != c)
{
int n1 = c - a;
int n2 = b - a;
int* i = a;
int* j = a + gcd_(n1, n2);
for ( ; i != j; i++ )
rotate_inner_(n1, n2, i, c);
}
return a + (c - b);
}
static void ip_merge_small_(int* a, int* b, int* c)
/* inplace merge.
* @note faster for small sequences. */
{
while ( a != b && b != c )
if (*a <= *b)
a++;
else
{
int* p = b+1;
while ( p != c && *p < *a )
p++;
rotate_(a, b, p);
b = p;
}
}
static void ip_merge_(int* a, int* b, int* c, int* t, const int ts)
/* inplace merge.
* @note works with or without additional memory. */
{
int n1 = b - a;
int n2 = c - b;
if (n1 <= n2 && n1 <= ts)
{
merge_(t, copy_(a, b, t), b, c, a);
}
else if (n2 <= ts)
{
merge_backward_(a, b, t, copy_(b, c, t), c);
}
/* merge without buffer. */
else if (n1 + n2 < 48)
{
ip_merge_small_(a, b, c);
}
else
{
int* p, * q;
if (n1 <= n2)
p = upper_bound_(a, b, *(q = b+n2/2));
else
q = lower_bound_(b, c, *(p = a+n1/2));
b = rotate_(p, b, q);
ip_merge_(a, p, b, t, ts);
ip_merge_(b, q, c, t, ts);
}
}
static void ip_merge_chunk_(const int cs, int* a, int* b, int* t,
const int ts)
{
int* p = a + cs*2;
for ( ; p <= b; a = p, p += cs*2 )
ip_merge_(a, a+cs, p, t, ts);
if (a+cs < b)
ip_merge_(a, a+cs, b, t, ts);
}
static void smallsort_(int* a, int* b)
/* insertion sort.
* @note any stable sort with low setup cost will do. */
{
int* p, * q;
for ( p = a+1; p < b; p++ )
{
int x = *p;
for ( q = p; a < q && x < *(q-1); q-- )
*q = *(q-1);
*q = x;
}
}
static void smallsort_chunk_(const int cs, int* a, int* b)
{
int* p = a + cs;
for ( ; p <= b; a = p, p += cs )
smallsort_(a, p);
smallsort_(a, b);
}
static void mergesort_lower_(int* v, int n, int* t, const int ts)
{
int cs = 16;
smallsort_chunk_(cs, v, v+n);
for ( ; cs < n; cs *= 2 )
ip_merge_chunk_(cs, v, v+n, t, ts);
}
static void* get_buffer_(int size, int* final)
{
void* p = NULL;
while ( size != 0 && (p = malloc(size)) == NULL )
size /= 2;
*final = size;
return p;
}
void mergesort(int* v, int n)
{
/* @note buffer size may be in the range [0,(n+1)/2]. */
int request = (n+1)/2 * sizeof(int);
int actual;
int* t = (int*) get_buffer_(request, &actual);
/* @note allocation failure okay. */
int tsize = actual / sizeof(int);
mergesort_lower_(v, n, t, tsize);
free(t);
}
它确实不容易或不有效，我建议您不要这样做，除非您真的必须这样做(您可能不必这样做，除非这是家庭作业，因为就地合并的应用程序主要是理论性的)。你不能用快速排序代替吗?无论如何，通过一些简单的优化，快速排序都会更快，而且它额外的内存是O(log N)。
不管怎样，如果你一定要做，那就必须做。这是我的发现:一和二。我不熟悉就地归并排序，但它的基本思想似乎是使用旋转来方便合并两个数组，而不使用额外的内存。
注意，这甚至比传统的归并排序还要慢。
我知道我来晚了，但这是我昨天写的一个解决方案。我也在其他地方发布了这个帖子，但这似乎是SO上最受欢迎的就地合并帖子。我也没有在其他地方看到这个算法，所以希望这能帮助到一些人。
这个算法是最简单的形式，所以它可以被理解。它可以显著地调整以获得额外的速度。平均时间复杂度为:稳定的原地阵列归并为O(n.log₂n)，整体排序为O(n.(log₂n)²)。
// Stable Merge In Place Sort
//
//
// The following code is written to illustrate the base algorithm. A good
// number of optimizations can be applied to boost its overall speed
// For all its simplicity, it does still perform somewhat decently.
// Average case time complexity appears to be: O(n.(log₂n)²)
#include <stddef.h>
#include <stdio.h>
#define swap(x, y) (t=(x), (x)=(y), (y)=t)
// Both sorted sub-arrays must be adjacent in 'a'
// Assumes that both 'an' and 'bn' are always non-zero
// 'an' is the length of the first sorted section in 'a', referred to as A
// 'bn' is the length of the second sorted section in 'a', referred to as B
static void
merge_inplace(int A[], size_t an, size_t bn)
{
int t, *B = &A[an];
size_t pa, pb; // Swap partition pointers within A and B
// Find the portion to swap. We're looking for how much from the
// start of B can swap with the end of A, such that every element
// in A is less than or equal to any element in B. This is quite
// simple when both sub-arrays come at us pre-sorted
for(pa = an, pb = 0; pa>0 && pb<bn && B[pb] < A[pa-1]; pa--, pb++);
// Now swap last part of A with first part of B according to the
// indicies we found
for (size_t index=pa; index < an; index++)
swap(A[index], B[index-pa]);
// Now merge the two sub-array pairings. We need to check that either array
// didn't wholly swap out the other and cause the remaining portion to be zero
if (pa>0 && (an-pa)>0)
merge_inplace(A, pa, an-pa);
if (pb>0 && (bn-pb)>0)
merge_inplace(B, pb, bn-pb);
} // merge_inplace
// Implements a recursive merge-sort algorithm with an optional
// insertion sort for when the splits get too small. 'n' must
// ALWAYS be 2 or more. It enforces this when calling itself
static void
merge_sort(int a[], size_t n)
{
size_t m = n/2;
// Sort first and second halves only if the target 'n' will be > 1
if (m > 1)
merge_sort(a, m);
if ((n-m)>1)
merge_sort(a+m, n-m);
// Now merge the two sorted sub-arrays together. We know that since
// n > 1, then both m and n-m MUST be non-zero, and so we will never
// violate the condition of not passing in zero length sub-arrays
merge_inplace(a, m, n-m);
} // merge_sort
// Print an array */
static void
print_array(int a[], size_t size)
{
if (size > 0) {
printf("%d", a[0]);
for (size_t i = 1; i < size; i++)
printf(" %d", a[i]);
}
printf("\n");
} // print_array
// Test driver
int
main()
{
int a[] = { 17, 3, 16, 5, 14, 8, 10, 7, 15, 1, 13, 4, 9, 12, 11, 6, 2 };
size_t n = sizeof(a) / sizeof(a[0]);
merge_sort(a, n);
print_array(a, n);
return 0;
} // main
包括它的“大结果”，本文描述了就地归并排序的几个变体(PDF):
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.22.5514&rep=rep1&type=pdf
少移动的就地排序
于尔基·卡塔贾宁、托米·
证明了n 元素可以使用O(1)进行排序 额外空间，O(n log n / log log n) 元素移动，nlog2n + O(nlog Log n)比较。这是第一次 需要就地排序算法 O (nlogn)在最坏情况下移动 同时保证O(n log n) 比较，不过由于不变 所涉及的因素是算法 主要是理论兴趣。
我认为这也是相关的。我有一份打印本，是同事传给我的，但我还没读过。它似乎涵盖了基本理论，但我对这个主题不够熟悉，无法判断它有多全面:
http://comjnl.oxfordjournals.org/cgi/content/abstract/38/8/681
最优稳定合并
安东尼奥斯·西姆沃尼斯
本文介绍了如何稳定地进行归并 两个序列A和B，大小为m和 n, m≤n，分别为O(m+n) 作业,O (mlog (n / m + 1)) 比较和只使用一个常数 额外空间的数量。这 结果匹配所有已知的下界…
利用c++ std::inplace_merge，就地归并排序可以实现如下:
template< class _Type >
inline void merge_sort_inplace(_Type* src, size_t l, size_t r)
{
if (r <= l) return;
size_t m = l + ( r - l ) / 2; // computes the average without overflow
merge_sort_inplace(src, l, m);
merge_sort_inplace(src, m + 1, r);
std::inplace_merge(src + l, src + m + 1, src + r + 1);
}
更多的排序算法，包括并行实现，可以在https://github.com/DragonSpit/ParallelAlgorithms repo中找到，它是开源的，是免费的。
