STL源码剖析之算法：prev_permutation

template <class BidirectionIterator>                                              
bool prev_permutation(BidirectionIterator first, BidirectionIterator last) {      
    if(first == last) return false;                                               
    BidirectionIterator i = first;                                                
    ++i;                                                                          
    if(i == last) return false;                                                   
    i = last;                                                                     
    --i;                                                                          
                                                                                  
    for(;;) {                                                                     
        BidirectionIterator ii = i;                                               
        --i;                                                                      
                                                                                  
        if(*ii < *i) {                                                            
            BidirectionIterator j = last;                                         
            while(!(*--j < *i));                                                  
            iter_swap(i, j);                                                      
            reverse(ii, last);                                                    
            return true;                                                          
        }                                                                         
                                                                                  
        if(i == first) {                                                          
            reverse(first, last);                                                 
            return false;                                                         
        }                                                                         
    }                                                                             
}