STL source code profiling algorithm stl_algo.h -- pre_permutation

STL source code profiling algorithm stl_algo.h -- pre_permutation

 

Pre_permutation

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Description: gets the first permutation and combination of the sequence marked by [first, last. If no value exists, false is returned. If yes, true is returned.
Ideas:
From the back to the front
1. Find two adjacent elements, set the left-side element to * I, and the right-side element to * ii, which meets * I> * ii
2. Find the first element smaller than * I and set it to * j. The * I and * j elements are reversed.
3. Reverse all elements on the right side of ii

Template
 
  
Bool prev_permutation (BidirectionalIterator first, BidirectionalIterator last) {if (first = last) return false; bidirealialiterator I = first; ++ I; if (I = last) return false; I = last; -- I; for (;) {BidirectionalIterator ii = I; -- I; if (* ii <* I) {// The condition for meeting * ii <* I --> next_permutation is * I <* ii BidirectionalIterator j = last; while (! (* -- J <* I); // when finding the * j <* I * j --> next_permutation, the condition is * I <* j iter_swap (I, j); reverse (ii, last); return true;} if (I = first) {reverse (first, last); return false ;}}}
 

Example:

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
 
  (cout,  ));  cout << endl;  prev_permutation(A, A+N);  cout << After prev_permutation: ;  copy(A, A+N, ostream_iterator
  
   (cout,  ));  cout << endl;  next_permutation(A, A+N);  cout << After next_permutation: ;  copy(A, A+N, ostream_iterator
   
    (cout,  ));  cout << endl;}