STL源码剖析——STL算法之permutation排列组合

前言

    由于在前文的《STL算法剖析》中，源码剖析非常多，不方便学习，也不方便以后复习，这里把这些算法进行归类，对他们单独的源码剖析进行讲解。本文讲解的是STL算法中的permutation排列组合算法，根据输入序列，排列出下一个排列组合或前一个排列组合。

permutation排列组合源码剖析

// next_permutation and prev_permutation, with and without an explicitly // supplied comparison function.//next_permutation获取[first,last)区间所标示序列的下一个排列组合，若果没有下一个排序组合，则返回false;否则返回true;/*函数功能：Rearranges the elements in the range [first,last) into the next lexicographically greater permutation.函数原型：default (1)：版本一采用less-than操作符template <class BidirectionalIterator>bool next_permutation (BidirectionalIterator first,                         BidirectionalIterator last);custom (2)：版本二采用仿函数comp决定template <class BidirectionalIterator, class Compare>bool next_permutation (BidirectionalIterator first,                         BidirectionalIterator last, Compare comp);*///版本一template <class _BidirectionalIter>bool next_permutation(_BidirectionalIter __first, _BidirectionalIter __last) {  __STL_REQUIRES(_BidirectionalIter, _BidirectionalIterator);  __STL_REQUIRES(typename iterator_traits<_BidirectionalIter>::value_type,                 _LessThanComparable);  if (__first == __last)    return false;//若为空，则返回false  _BidirectionalIter __i = __first;  ++__i;  if (__i == __last)//区间只有一个元素    return false;  //若区间元素个数不小于两个  __i = __last;//i指向尾端  --__i;//不断后移  for(;;) {  //下面两行是让ii和i成为相邻的元素  //其中i为第一个元素，ii为第二个元素    _BidirectionalIter __ii = __i;//    --__i;//以下在相邻元素判断    if (*__i < *__ii) {//若前一个元素小于后一个元素，//则再从最尾端开始往前检查，找出第一个大于*i的元素，令该元素为*j，将*i和*j交换//再将ii之后的所有元素颠倒排序      _BidirectionalIter __j = __last;//令j指向最尾端      while (!(*__i < *--__j))//由尾端往前检查，直到遇到比*i大的元素        {}      iter_swap(__i, __j);//交换迭代器i和迭代器j所指的元素      reverse(__ii, __last);//将ii之后的元素全部逆向重排      return true;    }    if (__i == __first) {//进行到最前面      reverse(__first, __last);//整个区间全部逆向重排      return false;    }  }}//版本二template <class _BidirectionalIter, class _Compare>bool next_permutation(_BidirectionalIter __first, _BidirectionalIter __last,                      _Compare __comp) {  __STL_REQUIRES(_BidirectionalIter, _BidirectionalIterator);  __STL_BINARY_FUNCTION_CHECK(_Compare, bool,    typename iterator_traits<_BidirectionalIter>::value_type,    typename iterator_traits<_BidirectionalIter>::value_type);  if (__first == __last)    return false;  _BidirectionalIter __i = __first;  ++__i;  if (__i == __last)    return false;  __i = __last;  --__i;  for(;;) {    _BidirectionalIter __ii = __i;    --__i;    if (__comp(*__i, *__ii)) {      _BidirectionalIter __j = __last;      while (!__comp(*__i, *--__j))        {}      iter_swap(__i, __j);      reverse(__ii, __last);      return true;    }    if (__i == __first) {      reverse(__first, __last);      return false;    }  }}//next_permutation函数举例：/*#include <iostream>     // std::cout#include <algorithm>    // std::next_permutation, std::sortint main () {  int myints[] = {1,2,3,4};  std::sort (myints,myints+4);  std::cout << "The 3! possible permutations with 3 elements:\n";  do {std::cout << myints[0] << ' ' << myints[1] << ' ' << myints[2] <<' ' << myints[3]<< '\n';  } while ( std::next_permutation(myints,myints+4) );   //std::next_permutation(myints,myints+4);  std::cout << "After loop: " << myints[0] << ' ' << myints[1] << ' ' << myints[2] << ' ' << myints[3]<<'\n';  return 0;}Output:The 3! possible permutations with 3 elements:1 2 3 41 2 4 31 3 2 41 3 4 21 4 2 31 4 3 22 1 3 42 1 4 32 3 1 42 3 4 12 4 1 32 4 3 13 1 2 43 1 4 23 2 1 43 2 4 13 4 1 23 4 2 14 1 2 34 1 3 24 2 1 34 2 3 14 3 1 24 3 2 1After loop: 1 2 3 4*///prev_permutation获取[first,last)区间所标示序列的上一个排列组合，若果没有上一个排序组合，则返回false;否则返回true;/*函数功能：Rearranges the elements in the range [first,last) into the previous lexicographically-ordered permutation.函数原型：default (1)：版本一采用less-than操作符template <class BidirectionalIterator>bool prev_permutation (BidirectionalIterator first,                         BidirectionalIterator last );custom (2)：版本二采用仿函数comptemplate <class BidirectionalIterator, class Compare>bool prev_permutation (BidirectionalIterator first,                         BidirectionalIterator last, Compare comp);*///版本一template <class _BidirectionalIter>bool prev_permutation(_BidirectionalIter __first, _BidirectionalIter __last) {  __STL_REQUIRES(_BidirectionalIter, _BidirectionalIterator);  __STL_REQUIRES(typename iterator_traits<_BidirectionalIter>::value_type,                 _LessThanComparable);  if (__first == __last)    return false;//若区间为空，返回false  _BidirectionalIter __i = __first;  ++__i;  if (__i == __last)//区间只有一个元素    return false;//返回false  //若区间元素个数不小于两个  __i = __last;  --__i;  for(;;) {  //下面两行是让ii和i成为相邻的元素  //其中i为第一个元素，ii为第二个元素    _BidirectionalIter __ii = __i;    --__i;//以下在相邻元素判断    if (*__ii < *__i) {//若前一个元素大于后一个元素，//则再从最尾端开始往前检查，找出第一个小于*i的元素，令该元素为*j，将*i和*j交换//再将ii之后的所有元素颠倒排序      _BidirectionalIter __j = __last;//令j指向最尾端            while (!(*--__j < *__i))//由尾端往前检查，直到遇到比*i小的元素        {}      iter_swap(__i, __j); //交换迭代器i和迭代器j所指的元素      reverse(__ii, __last);//将ii之后的元素全部逆向重排      return true;    }    if (__i == __first) {//进行到最前面      reverse(__first, __last);//把区间所有元素逆向重排      return false;    }  }}//版本二template <class _BidirectionalIter, class _Compare>bool prev_permutation(_BidirectionalIter __first, _BidirectionalIter __last,                      _Compare __comp) {  __STL_REQUIRES(_BidirectionalIter, _BidirectionalIterator);  __STL_BINARY_FUNCTION_CHECK(_Compare, bool,    typename iterator_traits<_BidirectionalIter>::value_type,    typename iterator_traits<_BidirectionalIter>::value_type);  if (__first == __last)    return false;  _BidirectionalIter __i = __first;  ++__i;  if (__i == __last)    return false;  __i = __last;  --__i;  for(;;) {    _BidirectionalIter __ii = __i;    --__i;    if (__comp(*__ii, *__i)) {      _BidirectionalIter __j = __last;      while (!__comp(*--__j, *__i))        {}      iter_swap(__i, __j);      reverse(__ii, __last);      return true;    }    if (__i == __first) {      reverse(__first, __last);      return false;    }  }}//prev_permutation函数举例/*#include <iostream>     // std::cout#include <algorithm>    // std::next_permutation, std::sort, std::reverseint main () {  int myints[] = {1,2,3};  std::sort (myints,myints+3);  std::reverse (myints,myints+3);  std::cout << "The 3! possible permutations with 3 elements:\n";  do {std::cout << myints[0] << ' ' << myints[1] << ' ' << myints[2] << '\n';  } while ( std::prev_permutation(myints,myints+3) );  std::cout << "After loop: " << myints[0] << ' ' << myints[1] << ' ' << myints[2] << '\n';  return 0;}Output:The 3! possible permutations with 3 elements:3 2 13 1 22 3 12 1 31 3 21 2 3After loop: 3 2 1*/

参考资料：

《STL源码剖析》侯捷