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    std::prev_permutation

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
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    (C++17)
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    (C++11)
    (C++11)
    (C++11)
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    (until C++17)(C++11)
    (C++20)(C++20)
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    (C++17)

    Sorting and related operations
    Partitioning operations
    (C++11)    
    (C++11)

    Sorting operations
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    (on partitioned ranges)
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    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations


     
    Defined in header <algorithm> 
    template< class BidirIt >
bool prev_permutation( BidirIt first, BidirIt last );
    		(1) (constexpr since C++20) 
    template< class BidirIt, class Compare >
bool prev_permutation( BidirIt first, BidirIt last, Compare comp );
    		(2) (constexpr since C++20)
    See More

    Permutes the target range [first, last) into the lexicographically previous permutation. If the previous permutation does not exist, transforms the range into the lexicographically last permutation.

    1) The set of all permutations is ordered lexicographically with respect to operator<(until C++20)std::less{}(since C++20).
    2) The set of all permutations is ordered lexicographically with respect to comp.

    If the type of *first is not Swappable(until C++11)BidirIt is not ValueSwappable(since C++11), the behavior is undefined.

    Parameters

    first, last - the pair of iterators defining the target range
    comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns true if the first argument is less than the second.

    The signature of the comparison function should be equivalent to the following:

    bool cmp(const Type1& a, const Type2& b);

    While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
    The types Type1 and Type2 must be such that an object of type BidirIt can be dereferenced and then implicitly converted to both of them.

    Type requirements
    -
    BidirIt must meet the requirements of ValueSwappable and LegacyBidirectionalIterator.

    Return value

    true if the target range is permuted into the previous permutation, otherwise false.

    Complexity

    Given N as std::distance(first, last):

    1,2) At most
    N
    2
     swaps.

    Exceptions

    Any exceptions thrown from iterator operations or the element swap.

    Notes

    Averaged over the entire sequence of permutations, typical implementations use about 3 comparisons and 1.5 swaps per call.

    Implementations (e.g. MSVC STL) may enable vectorization when the iterator type satisfies LegacyContiguousIterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

    Possible implementation

    template<class BidirIt>
bool prev_permutation(BidirIt first, BidirIt last)
{
    if (first == last)
        return false;
    BidirIt i = last;
    if (first == --i)
        return false;
    
    while (1)
    {
        BidirIt i1, i2;
        
        i1 = i;
        if (*i1 < *--i)
        {
            i2 = last;
            while (!(*--i2 < *i))
                ;
            std::iter_swap(i, i2);
            std::reverse(i1, last);
            return true;
        }
        
        if (i == first)
        {
            std::reverse(first, last);
            return false;
        }
    }
}

    Example

    The following code prints all six permutations of the string "cab" in reverse order.

    Run this code
    #include <algorithm>
#include <iostream>
#include <string>

int main()
{
    std::string s = "cab";
    
    do
    {
        std::cout << s << ' ';
    }
    while (std::prev_permutation(s.begin(), s.end()));
    
    std::cout << s << '\n';
}

    Output: 

    cab bca bac acb abc cba

    See also

    (C++20)
    		 generates the next smaller lexicographic permutation of a range of elements
    (algorithm function object)[edit]
    (C++11)
    		 determines if a sequence is a permutation of another sequence
    (function template & algorithm function object)[edit]
    (C++20)
    		generates the next greater lexicographic permutation of a range of elements
    (function template & algorithm function object)[edit]
    (C++20)