STL Source Code Analysis algorithm Stl_algo.h--pre_permutation

This article Senlie original, reproduced please retain this address:Http://blog.csdn.net/zhengsenlie

Pre_permutation

----------------------------------------------------------------
Descriptive narrative: The previous permutation of the sequence marked by [first, last].

Assume no, return false, have, return true
Ideas:
From the back forward
1. Find two adjacent elements, so that the left side of the element is *i, the right side of the element is *ii, and meet *i > *ii
2. Find the first element that is less than *i and make it *j. Swap the *I,*J elements
3. Invert all elements in the right side of II

Template <class Bidirectionaliterator>bool prev_permutation (bidirectionaliterator first,                      Bidirectionaliterator last) {  if (first = = last) return false;  Bidirectionaliterator i = first;  ++i;  if (i = = last) return false;  i = last;  I.;  for (;;) {    Bidirectionaliterator II = i;    I.;    if (*ii < *i) {  //Meet *ii < *i-Next_permutation condition is *i < *ii      Bidirectionaliterator j = Last;
   while (! ( *--j < *i)); Find the *j to meet *j < *i when the condition is *i < *j      Iter_swap (i, j);      Reverse (ii, last);      return true;    }    if (i = = first) {      reverse (first, last)      ; return false;}}  }

Demo Sample:

int main () {  int a[] = {2, 3, 4, 5, 6, 1};  const int N = sizeof (A)/sizeof (int);  cout << "initially:              ";  Copy (A, A+n, ostream_iterator<int> (cout, ""));  cout << Endl;  Prev_permutation (A, a+n);  cout << "after Prev_permutation:";  Copy (A, A+n, ostream_iterator<int> (cout, ""));  cout << Endl;  Next_permutation (A, a+n);  cout << "after Next_permutation:";  Copy (A, A+n, ostream_iterator<int> (cout, ""));  cout << Endl;}

Copyright notice: This article Bo Master original articles, blogs, without consent may not be reproduced.

STL Source Code Analysis algorithm Stl_algo.h--pre_permutation