How to Find Last Occurrence of an Element in a List in C++?

In C++, std::list represents a doubly linked list and the last occurrence of an element in a list refers to the last time that element appears in the list when traversed from the beginning. In this article, we will learn how to find the last occurrence of a specific element in a list in C++.

Example:

Input: 
list<int> myList = {1, 2, 7, 4, 5, 7, 8}
int key = 7;

Output: 
The last occurrence of element 7 is found at index :  5

Find the Last Occurrence of an Element in a List in C++

To find the last occurrence of an element in a std::list, we can use the std::find() function with reverse iterators std::list::rbegin() and std::list::rend() to iterate through the list in reverse order. When we iterate in reverse order, the first occurrence is the last occurrence of that element.

Approach

• Use the std::find function along with reverse iterators to get the iterator pointing to the first occurrence of the target element from the last.
• If the element is found calculate its index using the std::distance function. Pass the base iterator (which points to one position past the end of the list in a forward direction) and the begin iterator to this function.
• Since list indexing is zero-based, subtract one from the result obtained from the std::distance function to get the correct index.
• Finally, print the index which denotes the last occurrence of the specific element in the list.

C++ Program to Find the Last Occurrence of an Element in a List

The below example demonstrates how to find the last occurrence of a specific element in a given list in C++.

// C++ program to demonstrate how to find the last
// occurrence of a specific element in a given list
#include <algorithm>
#include <iostream>
#include <list>
using namespace std;

int main()
{
    // Initializing a list of integers
    list<int> myList = { 1, 2, 7, 4, 5, 7, 8 };

    // Declare the element whose last occurrence is to be
    // found
    int element = 7;

    // Finding the last occurrence of the element
    auto it = find(myList.rbegin(), myList.rend(), element);

    // Printing the position of the last occurrence of the
    // element
    if (it != myList.rend()) {
        // Finding the index of the last occurrence of the
        // element
        int position
            = distance(myList.begin(), it.base()) - 1;
        cout << "The last occurrence of element " << element
             << " is found at index : " << position << endl;
    }
    else {
        cout << "Element not found in the list" << endl;
    }
    return 0;
}

The last occurrence of element 7 is found at index : 5

Time Complexity: O(n), here n is the number of elements in the list.
Auxiliary Space: O(1)