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std::reverse() in C++
  • Difficulty Level : Easy
  • Last Updated : 17 Feb, 2021

reverse() is a predefined function in header file algorithm. It is defined as a template in the above mentioned header file. It reverses the order of the elements in the range [first, last) of any container. The time complexity is O(n). 
Note: The range used is [first, last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
Syntax: 

void reverse(BidirectionalIterator first, BidirectionalIterator last)
BidirectionalIterator is an iterator that can be used to access any
elements of a container in both forward and backward direction.

Examples: 

Input : 10 11 12 13 14 15 16 17
Output :10 11 12 13 14 17 16 15
Explanation:
reverse(v.begin() + 5, v.begin() + 8);
In the above function, input we have applied reverse() on the vector
from index 5 to index 7.
Therefore when we display the vector we get reverse order
from index 5 to index 7.

CPP




// CPP program to illustrate
// std::reverse() function of STL
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
  
int main()
{
    vector<int> vec1;
    vector<int>::iterator p;
  
    // Inserting elements in vector
    for (int i = 0; i < 8; i++) {
        vec1.push_back(i + 10);
    }
    // Displaying elements of vector
    cout<<"Initial Vector:"<<endl;
    for(p = vec1.begin(); p < vec1.end(); p++) {
        cout << *p << " ";
    }
    cout << endl;
  
    cout << "Reverse only from index 5 to 7 in vector:\n";
    // Reversing elements from index 5 to index 7
    reverse(vec1.begin() + 5, vec1.begin() + 8);
  
    // Displaying elements of vector after Reversing
    for (p = vec1.begin(); p < vec1.end(); p++) {
        cout << *p << " ";
    }
    cout << endl <<endl;
  
    vector<int> vec2{ 4, 5, 6, 7 };
  
    cout<<"Initial Vector:"<<endl;
    for (p = vec2.begin(); p < vec2.end(); p++) {
        cout << *p << " ";
    }
    cout << endl;
  
    cout << "Reverse full Vector:"<<endl;
    // Reversing directly from beginning to end
    reverse(vec2.begin(), vec2.end());
      
    // Displaying elements of vector after Reversing
    for (p = vec2.begin(); p < vec2.end(); p++) {
        cout << *p << " ";
    }
    cout << endl;
  
    return 0;
}

Output: 

Initial Vector:
10 11 12 13 14 15 16 17 
Reverse only from index 5 to 7 in vector:
10 11 12 13 14 17 16 15 

Initial Vector:
4 5 6 7 
Reverse full Vector:
7 6 5 4 

Time complexity: O(n)

This article is contributed by Hardik Gaur. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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