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Different Methods to Reverse a String in C++

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The reversing of a string is nothing but simply substituting the last element of a string to the 1st position of the string.

string-reverse

Different Methods to Reverse a String in C++ are:

  • Making our own reverse function 
  • Using ‘inbuilt’ reverse function 
  • Using Constructor
  • Using a temp file

1. Making a Custom Reverse Function For Swapping Characters

  •  Using a first to last approach ‘for’ loop

CPP

// C++ program to reverse a string
// using first to last approach
// 'for' loop
#include <bits/stdc++.h>
using namespace std;
 
// Function to reverse a string
void reverseStr(string& str)
{
    int n = str.length();
 
    // Swap character starting from two
    // corners
    for (int i = 0; i < n / 2; i++)
        swap(str[i], str[n - i - 1]);
}
 
// Driver program
int main()
{
    string str = "geeksforgeeks";
    reverseStr(str);
    cout << str;
    return 0;
}

                    

Output
skeegrofskeeg

Complexity Analysis:

Time Complexity: O(N)
Auxiliary Space: O(1)

  • Using a first to last Approach with while loop 

C++

// C++ program to reverse a string
// using while loop
#include <bits/stdc++.h>
using namespace std;
 
// Function to reverse a string
void reverseStr(string& str)
{
    int len = str.length();
    int n = len-1;
    int i = 0;
    while(i<=n){
        //Using the swap method to switch values at each index
        swap(str[i],str[n]);
        n = n-1;
        i = i+1;
  }
 
}
 
// Driver program
int main()
{
    string str = "geeksforgeeks";
    reverseStr(str);
    cout << str;
    return 0;
}

                    

Output
skeegrofskeeg

Complexity Analysis:

Time Complexity: O(N)
Auxiliary Space: O(1)

  •  Using a Last to First Approach ‘for‘ Loop  

C++

// C++ program to demonstrate reverse
// of a string using Last to First
// Approach 'for' Loop
#include <bits/stdc++.h>
using namespace std;
 
// Function to reverse a string
void reverse(string str)
{
    for (int i = str.length() - 1; i >= 0; i--)
        cout << str[i];
}
 
// Driver code
int main(void)
{
    string s = "GeeksforGeeks";
    reverse(s);
    return (0);
}

                    

Output
skeeGrofskeeG

Complexity Analysis:

Time Complexity: O(N)
Auxiliary Space: O(1)

  • Using a Last to First Approach ‘while’ Loop

C++

// C++ program to demonstrate reverse
// of a string using Last to First
// Approach 'while' Loop
#include <bits/stdc++.h>
using namespace std;
 
// Function to reverse a string
void reverse(string str)
{
    int len = str.length();
    int n = len;
    while(n--)
        cout << str[n];
}
 
// Driver code
int main(void)
{
    string s = "GeeksforGeeks";
 
    reverse(s);
    return (0);
}

                    

Output
skeeGrofskeeG

Complexity Analysis:

Time Complexity: O(N)
Auxiliary Space: O(1)

 

1. Using recursion Function with two pointer approach

Recursion functions are used for iterating to different indexes of the string.

C++

// C++ program to reverse a string
// using recursion
#include <bits/stdc++.h>
using namespace std;
 
// Function to reverse a string
void reverseStr(string& str, int n, int i)
{
     
  if(n<=i){return;}
//  Swapping the character
  swap(str[i],str[n]);
  reverseStr(str,n-1,i+1);
 
}
 
// Driver program
int main()
{
    string str = "geeksforgeeks";
    reverseStr(str, str.length()-1, 0);
    cout << str;
    return 0;
}

                    

Output
skeegrofskeeg

Complexity Analysis:

Time Complexity: O(N)

Auxiliary Space: O(N)

2. Using one pointer approach in recursion

Below is the implementation of the code:

C++

//C++ program to reverse a string using recursion
#include <iostream>
using namespace std;
void getreverse(string &str, int i)
{
    if (i > (str.length() - 1 - i))
    {
        return;
    }
    swap(str[i], str[str.length() - i - 1]);
    i++;
    getreverse(str, i);
}
int main()
{
    string name = "geeksforgeeks";
 
    getreverse(name, 0);
    cout << name << endl;
    return 0;
}
//code contributed by pragatikohli

                    

Output
skeegrofskeeg

Complexity Analysis:

Time Complexity: O(N)

Auxiliary Space: O(N)

3. Using the inbuilt “reverse” Function

There is a direct function in the “algorithm” header file for doing reverse that saves our time when programming.

// Reverses elements in [begin, end]
void reverse (BidirectionalIterator begin,
BidirectionalIterator end);

CPP

// C++ program to illustrate the
// reversing of a string using
// reverse() function
#include <bits/stdc++.h>
using namespace std;
int main()
{
    string str = "geeksforgeeks";
 
    // Reverse str[begin..end]
    reverse(str.begin(), str.end());
 
    cout << str;
    return 0;
}

                    

Output
skeegrofskeeg

Complexity Analysis:

Time Complexity: O(N)

Auxiliary Space: O(1)

4. Reverse a String Using the Constructor 

Passing reverse iterators to the constructor returns us a reversed string. 

CPP

// C++ program to reverse
// string using constructor
#include <bits/stdc++.h>
using namespace std;
int main()
{
    string str = "GeeksforGeeks";
 
    // Use of reverse iterators
    string rev = string(str.rbegin(), str.rend());
 
    cout << rev << endl;
    return 0;
}

                    

Output
skeeGrofskeeG

Complexity Analysis:

Time Complexity: O(N)

Auxiliary Space: O(1)

5. Using a Temporary String

CPP

// C++ program to demonstrate
// reversing of string
// using temporary string
#include <bits/stdc++.h>
using namespace std;
int main()
{
 
    string str = "GeeksforGeeks";
    int n = str.length();
   
    // Temporary string to store the reverse
    string rev;
   
    for (int i = n - 1; i >= 0; i--)
        rev.push_back(str[i]);
 
    cout << rev << endl;
    return 0;
}

                    

Output
skeeGrofskeeG

Complexity Analysis:

Time Complexity: O(N)

Auxiliary Space: O(1)

How could we get the reverse of a const string?

To get the reverse of a const string we have to first declare a ‘const string’ in a user-defined function following which we have declared then use the following algorithm for the calling of the desired objects.

“const reverseConstString = function(string) { return string.split("").reverse().join("")” 

Example: 

C++

// C++ program to get reverse of a const string
#include <bits/stdc++.h>
using namespace std;
 
// Function to reverse string and return
// reverse string pointer of that
char* reverseConstString(char const* str)
{
    // find length of string
    int n = strlen(str);
 
    // create a dynamic pointer char array
    char* rev = new char[n + 1];
 
    // copy of string to ptr array
    strcpy(rev, str);
 
    // Swap character starting from two
    // corners
    for (int i = 0, j = n - 1; i < j; i++, j--)
        swap(rev[i], rev[j]);
 
    // return pointer of the reversed string
    return rev;
}
 
// Driver code
int main(void)
{
    const char* s = "GeeksforGeeks";
    printf("%s", reverseConstString(s));
    return (0);
}

                    

Output
skeeGrofskeeG

Time Complexity: O(N)
Auxiliary Space: O(N)

Using Stack Data Structure

C++

// C++ Program to reverse a string
#include <bits/stdc++.h>
using namespace std;
int main()
{
    string s = "GeeksforGeeks";
    stack<char> st;
    for (char x : s)
        st.push(x);
    while (!st.empty()) {
        cout << st.top();
        st.pop();
    }
    return 0;
}

                    

Output
skeeGrofskeeG

Complexity Analysis:

Time Complexity: O(N)

Auxiliary Space: O(N)

Using Vector Data Structure

C++

// C++ Program to reverse a string
#include <bits/stdc++.h>
using namespace std;
int main()
{
    string s = "GeeksforGeeks";
    int n=s.length();
    vector<char> vec;
    for (int i = n - 1; i >= 0; i--)
        vec.push_back(s[i]);
     
    for(auto i:vec){
        cout<<i;
    }
    return 0;
}

                    

Output
skeeGrofskeeG

Complexity Analysis:

Time Complexity: O(N)

Auxiliary Space: O(N)

This article is contributed by Priyam kakati, Ranju Kumari, Somesh Awasthi.



Last Updated : 23 Sep, 2023
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