泛型算法系列26:inplace_merge()

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#merge #算法 #distance #iterator #class #each

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本文展示了一个C++实现的就地合并排序算法示例，该算法能够在不使用额外空间的情况下将两个已排序的序列合并成一个有序序列。通过具体的代码实例演示了如何对整型数组和向量进行排序，并提供了输出结果验证算法的有效性。

#include <algorithm> #include <vector> #include <iostream> #include <functional> using namespace std; /************************************************************************/ /* */ template<class _BidIt> inline void my_inplace_merge(_BidIt _First, _BidIt _Mid, _BidIt _Last) { // merge [_First, _Mid) with [_Mid, _Last), using operator< if (_First != _Mid && _Mid != _Last) _Inplace_merge(_CHECKED_BASE(_First), _CHECKED_BASE(_Mid), _CHECKED_BASE(_Last), _Dist_type(_First), _Val_type(_First)); } template<class _BidIt, class _Diff, class _Ty> inline void _Inplace_merge(_BidIt _First, _BidIt _Mid, _BidIt _Last, _Diff *, _Ty *) { // merge [_First, _Mid) with [_Mid, _Last), using operator< _DEBUG_ORDER(_First, _Mid); _DEBUG_ORDER(_Mid, _Last); _Diff _Count1 = 0; _Distance(_First, _Mid, _Count1); _Diff _Count2 = 0; _Distance(_Mid, _Last, _Count2); _Temp_iterator<_Ty> _Tempbuf(_Count1 < _Count2 ? _Count1 : _Count2); my_Buffered_merge(_First, _Mid, _Last, _Count1, _Count2, _Tempbuf); } template<class _BidIt, class _Diff, class _Ty> inline void my_Buffered_merge(_BidIt _First, _BidIt _Mid, _BidIt _Last, _Diff _Count1, _Diff _Count2, _Temp_iterator<_Ty>& _Tempbuf) { // merge [_First, _Mid) with [_Mid, _Last), using operator< if (_Count1 + _Count2 == 2) { // order two one-element partitions if (_DEBUG_LT(*_Mid, *_First)) std::iter_swap(_First, _Mid); } else if (_Count1 <= _Count2 && _Count1 <= _Tempbuf._Maxlen()) { // buffer left partition, then merge _STDEXT unchecked_copy(_First, _Mid, _Tempbuf._Init()); _STDEXT unchecked_merge(_Tempbuf._First(), _Tempbuf._Last(), _Mid, _Last, _First); } else if (_Count2 <= _Tempbuf._Maxlen()) { // buffer right partition, then merge _STDEXT unchecked_copy(_Mid, _Last, _Tempbuf._Init()); _STDEXT _Unchecked_merge_backward(_First, _Mid, _Tempbuf._First(), _Tempbuf._Last(), _Last); } else { // buffer too small, divide and conquer _BidIt _Firstn, _Lastn; _Diff _Count1n, _Count2n; if (_Count2 < _Count1) { // left larger, cut it in half and partition right to match _Count1n = _Count1 / 2, _Count2n = 0; _Firstn = _First; std::advance(_Firstn, _Count1n); _Lastn = std::lower_bound(_Mid, _Last, *_Firstn); _Distance(_Mid, _Lastn, _Count2n); } else { // right larger, cut it in half and partition left to match _Count1n = 0, _Count2n = _Count2 / 2; _Lastn = _Mid; std::advance(_Lastn, _Count2n); _Firstn = std::upper_bound(_First, _Mid, *_Lastn); _Distance(_First, _Firstn, _Count1n); } _BidIt _Midn = _Buffered_rotate(_Firstn, _Mid, _Lastn, _Count1 - _Count1n, _Count2n, _Tempbuf); // rearrange middle my_Buffered_merge(_First, _Firstn, _Midn, _Count1n, _Count2n, _Tempbuf); // merge each new part my_Buffered_merge(_Midn, _Lastn, _Last, _Count1 - _Count1n, _Count2 - _Count2n, _Tempbuf); } } /************************************************************************/ template <class Type> void print_elements( Type elem ) { cout << elem << " "; } /* * generates: ia sorted into two subarrays: 12 15 17 20 23 26 29 35 40 51 10 16 21 41 44 54 62 65 71 74 ia inplace_merge: 10 12 15 16 17 20 21 23 26 29 35 40 41 44 51 54 62 65 71 74 ivec sorted into two subvectors: 51 40 35 29 26 23 20 17 15 12 74 71 65 62 54 44 41 21 16 10 ivec inplace_merge: 74 71 65 62 54 51 44 41 40 35 29 26 23 21 20 17 16 15 12 10 */ int main() { int ia[] = { 29,23,20,17,15,26,51,12,35,40, 74,16,54,21,44,62,10,41,65,71 }; vector< int > ivec( ia, ia + 20 ); void (*pfi)( int ) = print_elements; // place the two subsequences in sorted order sort( &ia[0], &ia[10] ); sort( &ia[10], &ia[20] ); cout << "ia sorted into two sub-arrays: /n"; for_each( ia, ia+20, pfi ); cout << "/n/n"; inplace_merge( ia, ia + 10, ia + 20 ); cout << "ia inplace_merge:/n"; for_each( ia, ia + 20, pfi ); cout << "/n/n"; sort( ivec.begin(), ivec.begin() + 10, greater<int>() ); sort( ivec.begin()+10, ivec.end(), greater<int>() ); cout << "ivec sorted into two sub-vectors: /n"; for_each( ivec.begin(), ivec.end(), pfi ); cout << "/n/n"; inplace_merge( ivec.begin(), ivec.begin() + 10, ivec.end(), greater<int>() ); cout << "ivec inplace_merge:/n"; for_each( ivec.begin(), ivec.end(), pfi ); cout << endl; return 0; }