C++按以下顺序切换数组顺序的算法:[Last,First,Last - 1,First + 1 ...]

Question

如何仅使用循环实现此类功能？

我正在打破我的脑袋,我似乎无法直接思考.

这就是我想出来的,但它甚至都没有.

for (int i = 0; i < elements; i++)
{
    for (int n = elements; n > 0; --n)
        a[i] = b[n];
}

Answer 1

这个给你

#include <iostream>
#include <algorithm>
#include <iterator>

int main()
{
    int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    for ( int x : a ) std::cout << x << ' ';
    std::cout << std::endl;

    for ( auto it = std::begin( a ); it != std::end( a ); it == std::end( a ) ? it : ++it )
    {
        it = std::rotate( it, std::prev( std::end( a ) ), std::end( a ) );
    }        

    for ( int x : a ) std::cout << x << ' ';
    std::cout << std::endl;
}        

程序输出是

0 1 2 3 4 5 6 7 8 9 
9 0 8 1 7 2 6 3 5 4 

编译器应支持C++ 11算法std::rotate.

PS我改变了循环的第三个表达式,它对于具有奇数个元素的序列是正确的.

另一种方法是使用标准算法std::copy_backward ,如下所示

#include <iostream>
#include <algorithm>
#include <iterator>

template <class BidirectionalIterator>
void alternate( BidirectionalIterator first, BidirectionalIterator last )
{
    if ( first != last && first != --last )
    {
        while ( first != last )
        {
            auto value = *last;
            std::copy_backward( first, last, std::next( last ) );
            *first++ = value;
            if ( first != last ) ++first;
        }
    }
}    

int main()
{
    int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    for ( int x : a ) std::cout << x << ' ';
    std::cout << std::endl;

    alternate( std::begin( a ), std::end( a ) );

    for ( int x : a ) std::cout << x << ' ';
    std::cout << std::endl;
}        

程序输出是

0 1 2 3 4 5 6 7 8 9 
9 0 8 1 7 2 6 3 5 4