How to traverse a STL map in reverse direction?

Map stores the elements in sorted order of keys. Now if we want to traverse it in reverse order we will use reverse_iterator of map. 

Syntax:

map::reverse_iterator iterator_name;

Reverse Iterator of map moves in backward direction on increment. So, we will point the reverse_iterator to the last element of map and then keep on incrementing it until it reaches the first element. To do this we will use 2 member functions of std::map i.e. 
1. rbegin() : It returns the reverse_iterator pointing to last element of map. 
2. rend() : It returns the reverse_iterator pointing to first element of map.

Now for traversing in reverse order we will iterate over the range b/w rbegin() & rend() using reverse_iterator. 

Reverse Iteration in map:

Example:

Input:  (10, "geeks"), (20, "practice"),  (5, " contribute")
Output : (20, "practice"),  (10, "geeks"), (5, " contribute")

CPP

// C++ program makes a map to iterate
// elements in reverse order.
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // Creating & Initializing a map of String & Ints
    map<int, string> mymap;

    // Inserting the elements one by one
    mymap.insert(make_pair(10, "geeks"));
    mymap.insert(make_pair(20, "practice"));
    mymap.insert(make_pair(5, "contribute"));

    // Create a map reverse iterator
    map<int, string>::reverse_iterator it;

    // rbegin() returns to the last value of map
    for (it = mymap.rbegin(); it != mymap.rend(); it++) {
        cout << "(" << it->first << ", "
             << it->second << ")" << endl;
    }

    return 0;
}

Output:

(20, practice)
(10, geeks)
(5, contribute)

We can also use auto to avoid remembering complex syntax. 

CPP

// C++ program makes a map to iterate
// elements in reverse order with simpler
// syntax
#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Creating & Initializing a map of String & Ints
    map<int, string> mymap;

    // Inserting the elements one by one
    mymap.insert(make_pair(10, "geeks"));
    mymap.insert(make_pair(20, "practice"));
    mymap.insert(make_pair(5, "contribute"));

    // rbegin() returns to the last value of map
    for (auto it = mymap.rbegin(); it != mymap.rend(); it++) {
        cout << "(" << it->first << ", "
             << it->second << ")" << endl;
    }

    return 0;
}

Output:

(20, practice)
(10, geeks)
(5, contribute)

Refer end for complexity analysis.

Reverse Iteration Using map’s key_type Range:

Another method to traverse a map in reverse order is to use map’s key_type range, and traverse the keys in reverse order.

Example:

Input:  (15, "Geeks"), (25, "GFG"),  (10, "GeeksForGeeks")
Output : (25, "GFG"),  (15, "Geeks"), (10, "GeeksForGeeks")

Below is the implementation:

C++

// C++ program makes a map to iterate
// elements in reverse order
// using map's key_type range
#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Creating & Initializing a map of Ints & Strings
    map<int, string> mymap;

    // Inserting the elements one by one
    mymap.insert(make_pair(15, "Geeks"));
    mymap.insert(make_pair(25, "GFG"));
    mymap.insert(make_pair(10, "GeeksForGeeks"));

    // Print the content of the map in reverse order
    for (auto it = mymap.rbegin()->first;
         it >= mymap.begin()->first; --it) {
        if (mymap.count(it))
            cout << "(" << it << ", " << mymap[it] << ")"
                 << endl;
    }
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

(25, GFG)
(15, Geeks)
(10, GeeksForGeeks)

Refer end for complexity analysis.

Reverse Iteration in multimap:

Multimap is similar to map with an addition that multiple elements can have same keys. Rather than each element being unique, the key value and mapped value pair has to be unique in this case. 

Example:

Input :  (10, "geeks"), (20, "practice"),  (5, "contribute"), 
         (20, "van"), (20, "watch"), (5, "joker")
Output:  (20, "watch"), (20, "van"), (20, "practice"), 
         (10, "geeks"), (5, "joker"), (5, "contribute")

CPP

// C++ program makes a multimap to store
// elements in descending order.
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // Creating & Initializing a multimap
    // of Ints & String
    multimap<int, std::string> mymap;

    // Inserting the elements one by one
    mymap.insert(make_pair(10, "geeks"));
    mymap.insert(make_pair(20, "practice"));
    mymap.insert(make_pair(5, "contribute"));

    // Duplicates allowed
    mymap.insert(make_pair(20, "van"));

    mymap.insert(make_pair(20, "watch"));
    mymap.insert(make_pair(5, "joker"));

    for (auto it = mymap.rbegin(); it != mymap.rend(); it++) {
        cout << "(" << it->first << ", "
            << it->second << ")" << endl;
    }

    return 0;
}

Output:

(20, watch)
(20, van)
(20, practice)
(10, geeks)
(5, joker)
(5, contribute)

Refer end for complexity analysis.

Reverse Iteration in map without using rbegin() or rend() function:

We can use normal begin() and end() function to iterate the map in reverse order.
Example:

Input:  (15, "Geeks"), (25, "GFG"),  (10, "GeeksForGeeks")
Output : (25, "GFG"),  (15, "Geeks"), (10, "GeeksForGeeks")

Below is the implementation:

C++

// C++ program makes a map to iterate
// elements in reverse order
// without using map.rbegin() and map.rend() functions
#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Creating & Initializing a map of Ints & Strings
    map<int, string> mymap;

    // Inserting the elements one by one
    mymap.insert(make_pair(15, "Geeks"));
    mymap.insert(make_pair(25, "GFG"));
    mymap.insert(make_pair(10, "GeeksForGeeks"));

    // set iterator it as the end()
    // which returns to the last value of map
    auto it = mymap.end();
    it--;

    // Print the content of the map in reverse order
    while (it != mymap.begin()) {
        cout << "(" << it->first << ", " << it->second
             << ")" << endl;
        it--;
        if (it == mymap.begin())
            cout << "(" << it->first << ", " << it->second
                 << ")" << endl;
    }
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

(25, GFG)
(15, Geeks)
(10, GeeksForGeeks)

Refer end for complexity analysis.

Reverse Iteration in map using vector:

We can create a vector with all the elements of the map, then use the std::reverse algorithm to reverse the order of the elements in the vector, then iterate through the vector to access the elements in the reversed order.

Input:  (15, "Geeks"), (25, "GFG"),  (10, "GeeksForGeeks")
Output : (25, "GFG"),  (15, "Geeks"), (10, "GeeksForGeeks")

Below is the implementation:

C++

// C++ program makes a map to iterate
// elements in reverse order using vector

#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Creating & Initializing a map of Ints & Strings
    map<int, string> mymap;

    // Inserting the elements one by one
    mymap.insert(make_pair(15, "Geeks"));
    mymap.insert(make_pair(25, "GFG"));
    mymap.insert(make_pair(10, "GeeksForGeeks"));

    // Create a vector with all the elements of the map
    vector<pair<int, string> > v(mymap.begin(), mymap.end());

    // Reverse the order of the elements in the vector
    reverse(v.begin(), v.end());

    // Iterate through the vector to access the elements in
    // the reversed order
      for (auto it : v) {
        cout << "(" << it.first << ", " << it.second << ")" << endl;
    }
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

(25, GFG)
(15, Geeks)
(10, GeeksForGeeks)

Refer end for complexity analysis.

Reverse Iteration in map using cbegin() and cend() :

cend() and cbegin() are member functions of the map container in the C++ Standard Template Library. cbegin() returns an iterator pointing to the first element in the container, while cend() returns an iterator pointing to the position just after the last element in the container. By using these functions, we can traverse the elements of a map in reverse order.

Input:  (15, "Geeks"), (25, "GFG"),  (10, "GeeksForGeeks")
Output: (25, "GFG"),  (15, "Geeks"), (10, "GeeksForGeeks")

Below is the implementation:

C++

// C++ program makes a map to iterate
// elements in reverse order using cbegin() and cend()

#include <iostream>
#include <map>

using namespace std;

int main()
{
    // Creating & Initializing a map of Ints & Strings
    map<int, string> my_map;

    // Inserting the elements one by one
    my_map.insert(make_pair(15, "Geeks"));
    my_map.insert(make_pair(25, "GFG"));
    my_map.insert(make_pair(10, "GeeksForGeeks"));

    // Traverse the map in reverse direction
    for (auto it = my_map.cend(); it != my_map.cbegin();
         it--) {
        auto itr = it;
        itr--;
        cout << itr->first << " -> " << itr->second << endl;
    }

    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

25 -> GFG
15 -> Geeks
10 -> GeeksForGeeks

Refer end for complexity analysis.

Reverse Iteration in map using crbegin() and crend() :

crend() and crbegin() are member functions of the map container in the C++ Standard Template Library. They return constant reverse iterators to the last element and the first element of the container, respectively. By using these functions, we can traverse the elements of a map in reverse order.

Input:  (15, "Geeks"), (25, "GFG"),  (10, "GeeksForGeeks")
Output : (25, "GFG"),  (15, "Geeks"), (10, "GeeksForGeeks")

Below is the implementation:

C++

// C++ program makes a map to iterate
// elements in reverse order using crbegin() and crend()

#include <iostream>
#include <map>

using namespace std;

int main()
{
    // Creating & Initializing a map of Ints & Strings
    map<int, string> my_map;

    // Inserting the elements one by one
    my_map.insert(make_pair(15, "Geeks"));
    my_map.insert(make_pair(25, "GFG"));
    my_map.insert(make_pair(10, "GeeksForGeeks"));

    // Traverse the map in reverse direction
    for (auto it = my_map.crbegin(); it != my_map.crend();
         ++it) {
        cout << it->first << " -> " << it->second << endl;
    }

    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

25 -> GFG
15 -> Geeks
10 -> GeeksForGeeks

Refer end for complexity analysis.

Complexity Analysis:

Methods	Time Complexity	Auxiliary Space
Using reverse Iteration in map	O(N)	O(1)
Using map’s key_type Range	O(N)	O(1)
In multimap	O(N)	O(1)
Without using rbegin() or rend() function	O(N)	O(1)
Using vector	O(N)	O(N)
using cbegin() and cend()	O(N)	O(1)
using crbegin() and crend()	O(N)	O(1)