如何能标准:: make_heap实现最多3N的比较,同时使? [英] How can std::make_heap be implemented while making at most 3N comparisons?
问题描述
我看着到的C ++ 0x标准,发现make_heap应该做的不超过3 * N比较的要求。
I looked in to the C++0x standard and found the requirement that make_heap should do no more than 3*N comparisons.
即。 heapify的无序集合可以在O(N)来完成
I.e. heapify an unordered collection can be done in O(N)
/* @brief Construct a heap over a range using comparison functor.
这是为什么?
Why is this?
该人士没有给我的线索(G ++ 4.4.3)
The source gives me no clues (g++ 4.4.3)
的,而(真)+ __parent == 0并不线索,而是代表O猜测(N)的行为
The while (true) + __parent == 0 are not clues but rather a guess for O(N) behaviour
template<typename _RandomAccessIterator, typename _Compare>
void
make_heap(_RandomAccessIterator __first, _RandomAccessIterator __last,
_Compare __comp)
{
const _DistanceType __len = __last - __first;
_DistanceType __parent = (__len - 2) / 2;
while (true)
{
_ValueType __value = _GLIBCXX_MOVE(*(__first + __parent));
std::__adjust_heap(__first, __parent, __len, _GLIBCXX_MOVE(__value),
__comp);
if (__parent == 0)
return;
__parent--;
}
}
__ adjust_heap看起来像一个日志N法:
__adjust_heap looks like a log N method:
while ( __secondChild < (__len - 1) / 2)
{
__secondChild = 2 * (__secondChild + 1);
是一个沼泽标准日志N至我。
Is a bog standard log N to me.
template<typename _RandomAccessIterator, typename _Distance,
typename _Tp, typename _Compare>
void
__adjust_heap(_RandomAccessIterator __first, _Distance __holeIndex,
_Distance __len, _Tp __value, _Compare __comp)
{
const _Distance __topIndex = __holeIndex;
_Distance __secondChild = __holeIndex;
while (__secondChild < (__len - 1) / 2)
{
__secondChild = 2 * (__secondChild + 1);
if (__comp(*(__first + __secondChild),
*(__first + (__secondChild - 1))))
__secondChild--;
*(__first + __holeIndex) = _GLIBCXX_MOVE(*(__first + __secondChild));
__holeIndex = __secondChild;
}
if ((__len & 1) == 0 && __secondChild == (__len - 2) / 2)
{
__secondChild = 2 * (__secondChild + 1);
*(__first + __holeIndex) = _GLIBCXX_MOVE(*(__first
+ (__secondChild - 1)));
__holeIndex = __secondChild - 1;
}
std::__push_heap(__first, __holeIndex, __topIndex,
_GLIBCXX_MOVE(__value), __comp);
}
任何线索,这是为什么O&LT; pciated = 3N将AP $ P $
编辑:
Any clues to why this is O <= 3N will be appreciated.
实验结果:
这实际实现使用
- &LT; 2N为heapifying堆比较
- &LT; 1.5N在相反的顺序heapifying堆。
推荐答案
@templatetypedef已经给出的一个很好的回答为什么 build_heap
是 O(N)的。也有在 CLRS 时,第二版第6章一个证明。
@templatetypedef has already given a good answer for why the asymptotic run time of build_heap
is O(n). There is also a proof in chapter 6 of CLRS, 2nd edition.
至于为什么C ++标准要求,在大多数的 3N 的比较是使用:
As for why the C++ standard requires that at most 3n comparisons are used:
这是我的实验(见下文code),看来确实低于的 2N 的需要比较。事实上,这些讲义包含一个证明 build_heap
只使用 2(N-⌈logn⌉)的比较。
From my experiments (see code below), it appears that actually less than 2n comparisons are needed. In fact, these lecture notes contain a proof that build_heap
only uses 2(n-⌈log n⌉) comparisons.
绑定的从标准似乎比需要更多的慷慨。
The bound from the standard seems to be more generous than required.
def parent(i):
return i/2
def left(i):
return 2*i
def right(i):
return 2*i+1
def heapify_cost(n, i):
most = 0
if left(i) <= n:
most = 1 + heapify_cost(n, left(i))
if right(i) <= n:
most = 1 + max(most, heapify_cost(n, right(i)))
return most
def build_heap_cost(n):
return sum(heapify_cost(n, i) for i in xrange(n/2, 1, -1))
的一些结果:
n 10 20 50 100 1000 10000
build_heap_cost(n) 9 26 83 180 1967 19960
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