How to traverse through all values for a given key in multimap?

Given a multimap and a key of the multimap, our task is to simply display the (key – value) pairs of the given key. In multimap we can have multiple (key – value) pair for the same key. Suppose our multimap contains 

key     value
1         10
2         20
2         30
2         40
3         50
4         60
4         70

key : 2
key     value
2         20
2         30
2         40

Like in unordered_map in C++ STL we cant fetch values like 

int key = 2;
multimap  map;

// insert values in map
cout << "Key : " << key;
cout << "Value : " < second;

Output : 

Key : 2
Value : 20

Because the above method will only return the first occurrence of the key present, This method fails if there are multiple (key – value) pairs for the same key.
There are two ways by which we can achieve the expected results : 
Method 1 (Simple Traversal) Traverse through whole map and whenever the key is equal to given key we display the key-value pair. 

C++

// CPP program to find all values for a
// given key.
#include <bits/stdc++.h>
using namespace std;

int main()
{
    multimap <int, int> map;

    // insert the values in multimap
    map.insert(make_pair(1, 10));
    map.insert(make_pair(2, 20));
    map.insert(make_pair(2, 30));
    map.insert(make_pair(2, 40));
    map.insert(make_pair(3, 50));
    map.insert(make_pair(4, 60));
    map.insert(make_pair(4, 70));

    int key = 2;
    for (auto itr = map.begin(); itr != map.end(); itr++)
        if (itr -> first == key)
            cout << itr -> first << "  "
                 << itr -> second << endl;

    return 0;
}

Java

// JAVA program to find all values for a
// given key.
import java.util.*;

class GFG
{

static class pair
{
    int first, second;
    public pair(int first, int second)
    {
        this.first = first;
        this.second = second;
    }
}

public static void main(String[] args)
{
    HashSet <pair> map = new LinkedHashSet<>();

    // add the values in multimap
    map.add(new pair(1, 10));
    map.add(new pair(2, 20));
    map.add(new pair(2, 30));
    map.add(new pair(2, 40));
    map.add(new pair(3, 50));
    map.add(new pair(4, 60));
    map.add(new pair(4, 70));

    int key = 2;
    for (pair itr : map)
        if (itr.first == key)
            System.out.println(itr.first+ " "
                + itr.second);
}
}

// This code is contributed by 29AjayKumar

Python3

# Python program to find all values for a
# given key.
map = []

# insert the values in multimap
map.append((1, 10));
map.append((2, 20));
map.append((2, 30));
map.append((2, 40));
map.append((3, 50));
map.append((4, 60));
map.append((4, 70));

key = 2;
for i in map:
    if i[0] == key:
        print(i[0],i[1])

# This code is contributed by shubhamsingh10

C#

// C# program to find all values for a
// given key.
using System;
using System.Collections.Generic;

class GFG
{

class pair
{
    public int first, second;
    public pair(int first, int second)
    {
        this.first = first;
        this.second = second;
    }
}

// Driver code
public static void Main(String[] args)
{
    HashSet<pair> map = new HashSet<pair>();

    //.Add the values in multimap
    map.Add(new pair(1, 10));
    map.Add(new pair(2, 20));
    map.Add(new pair(2, 30));
    map.Add(new pair(2, 40));
    map.Add(new pair(3, 50));
    map.Add(new pair(4, 60));
    map.Add(new pair(4, 70));

    int key = 2;
    foreach (pair itr in map)
        if (itr.first == key)
            Console.WriteLine(itr.first+ " "
                + itr.second);
}
}

// This code is contributed by Rajput-Ji

Javascript

<script>

class pair
{
    constructor(first,second)
    {
        this.first=first;
        this.second=second;
    }
}

let map = new Set();
// add the values in multimap
map.add(new pair(1, 10));
map.add(new pair(2, 20));
map.add(new pair(2, 30));
map.add(new pair(2, 40));
map.add(new pair(3, 50));
map.add(new pair(4, 60));
map.add(new pair(4, 70));

let key = 2;
for (let itr of map.values())
    if (itr.first == key)
        document.write(itr.first+ " "
                   + itr.second+"<br>");


// This code is contributed by rag2127
</script>

Output:

2  20
2  30
2  40

Method 2(Using Binary Search)Find the lower_bound and upper_bound for the given key and traverse between them. 
lower_bound(key) : returns the iterator pointing to the first element which is greater than or 
equal to key. 
upper_bound(key) : returns the iterator pointing to the first element which is greater than key. 

key     value
1         10
2         20 <-- lower_bound(20)
2         30
2         40
3         50 <-- upper_bound(20)
4         60
4         70

CPP

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

int main()
{
    multimap <int, int> map;

    // insert the values in multimap
    map.insert(make_pair(1, 10));
    map.insert(make_pair(2, 20));
    map.insert(make_pair(2, 30));
    map.insert(make_pair(2, 40));
    map.insert(make_pair(3, 50));
    map.insert(make_pair(4, 60));
    map.insert(make_pair(4, 70));

    int key = 2;
    auto itr1 = map.lower_bound(key);
    auto itr2 = map.upper_bound(key);

    while (itr1 != itr2)
    {
        if (itr1 -> first == key)
           cout << itr1 -> first << "  "
                << itr1 -> second << endl;
        itr1++;
    }
    return 0;
}

Output: 

2  20
2  30
2  40

Time complexity: O(logn) where n is size of given multimap

Auxiliary Space: O(n)