STL 源码剖析 算法 stl_algo.h -- rotate

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rotate
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描述：将[first, middle) 内的元素和[middle, last) 内的元素互换。

/*------------------------------------------------------------*分派函数(dispatch function)*/template <class ForwardIterator>inline void rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last){if(first == middle || middle == last) return ;__rotate(first, middle, last, distance_type(first), iterator_category(first));}

下面根据不同的迭代器类型完成三个旋转操作

rotate 的 forward iterator 版
思路：
1.以短段为依据，对应元素逐一对调
2.调整，对新的前、后段再作交换
3.当两段都结束时，循环结束

复杂度：O(n)

template<class ForwardIterator, class Distance>void __rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last, Distance*, forward_iterator_tag){for(ForwardIterator i = middle ; ; ){iter_swap(first, i); //前段，后段的元素一一交换++first; ++i;  // 双双前进 1// 以下判断是前段[first, middle)先结束还是后段 [middle, last) 后结束if(first == middle){ //前段结束了if(i == last) return; //如果后段同时也结束，整个就结束了middle = i;}else if(i == last){ //后段先结束i = middle; // 调整，准备对新的的前、后段再作交换}}}

rotate 的 bidirectional iterator 版
思路：
1.对前段逆转
2.对后段逆转
3.对整体逆转
复杂度：O(n)

template<class BidirectionalIterator, class Distance>void __rotate(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last, Distance *,bidirectional_iterator_tag){reverse(first, middle);reverse(middle,last);reverse(first, last);}

rotate 的 random access iterator 版 --> ?? 看不太懂。。。

template<class RandomAccessIterator, class Distance>void __rotate(RandomAccessIterator first, RandomAccessIterator middle，RandomAccessIterator last, Distance *,random_access_iterator_tag){//取全长和前段长度的最大公因子Distance n = __gcd(last - first, middle - first);while(n--){__rotate_cycle(first, last, first + n, middle - first, value_type(first));}}template <class EuclideanRingElement>EuclideanRingElement __gcd(EuclideanRingElement m, EuclideanRingElement n){  while (n != 0) {    EuclideanRingElement t = m % n;    m = n;    n = t;  }  return m;}template <class RandomAccessIterator, class Distance, class T>void __rotate_cycle(RandomAccessIterator first, RandomAccessIterator last,                    RandomAccessIterator initial, Distance shift, T*) {  T value = *initial;  RandomAccessIterator ptr1 = initial;  RandomAccessIterator ptr2 = ptr1 + shift;  while (ptr2 != initial) {    *ptr1 = *ptr2;    ptr1 = ptr2;    if (last - ptr2 > shift)      ptr2 += shift;    else      ptr2 = first + (shift - (last - ptr2));  }  *ptr1 = value;}

示例：

char alpha[] = "abcdefghijklmnopqrstuvwxyz";rotate(alpha, alpha + 13, alpha + 26);printf("%s\n", alpha);// The output is nopqrstuvwxyzabcdefghijklm