STL 源码剖析 算法 stl_algo.h -- merge sort

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merge sort

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描述：归并排序

思路：

1.将区间对半分割

2.对左、右段分别排序

3.利用inplace_merge将左、右段合并成为一个完整的有序序列

复杂度：O(nlog n)

源码：

template<class BidirectionalIter>
void mergesort(BidirectionalIter first, BidirectionalIter last){
typename iterator_traits<BidirectionalIter>::diference_type n = distance(first,last);
if(n == 0 || n == 1) return ;
else{
BidirectionalIter mid = first + n / 2;
mergesort(first, mid);
mergesort(mid, last);
inplace_merge(first, mid, last);
}
}

示例：

int main()
{

int a[]={3,8,0,6,7,4,2,1,9,3,1,8,3,9,2,0,9};
int *a_end=a+sizeof a/sizeof(int);

std::cout<<"a before mergesort: ";
std::for_each(a, a_end, print<int>);
std::cout<<'\n';

mergesort(a, a_end);

std::cout<<"a after mergesort: ";
std::for_each(a, a_end, print<int>);
std::cout<<'\n';

return 0;
}