C++ Program for Left Rotation and Right Rotation of a String

Given a string of size n, write functions to perform the following operations on a string-

1. Left (Or anticlockwise) rotate the given string by d elements (where d <= n)
2. Right (Or clockwise) rotate the given string by d elements (where d <= n).

Examples: 

Input : s = "GeeksforGeeks"
        d = 2
Output : Left Rotation  : "eksforGeeksGe" 
         Right Rotation : "ksGeeksforGee"  


Input : s = "qwertyu" 
        d = 2
Output : Left rotation : "ertyuqw"
         Right rotation : "yuqwert"

Method 1:

A Simple Solution is to use a temporary string to do rotations. For left rotation, first, copy last n-d characters, then copy first d characters in order to the temporary string. For right rotation, first, copy last d characters, then copy n-d characters. 

Can we do both rotations in-place and O(n) time? 
The idea is based on a reversal algorithm for rotation.

// Left rotate string s by d (Assuming d <= n)
leftRotate(s, d)
  reverse(s, 0, d-1); // Reverse substring s[0..d-1]
  reverse(s, d, n-1); // Reverse substring s[d..n-1]
  reverse(s, 0, n-1); // Reverse whole string.  

// Right rotate string s by d (Assuming d <= n)
rightRotate(s, d)

  // We can also call above reverse steps
  // with d = n-d.
  leftRotate(s, n-d)

Below is the implementation of the above steps : 

// C program for Left Rotation and Right
// Rotation of a String
#include<bits/stdc++.h>
using namespace std;

// In-place rotates s towards left by d
void leftrotate(string &s, int d)
{
    reverse(s.begin(), s.begin()+d);
    reverse(s.begin()+d, s.end());
    reverse(s.begin(), s.end());
}

// In-place rotates s towards right by d
void rightrotate(string &s, int d)
{
   leftrotate(s, s.length()-d);
}

// Driver code
int main()
{
    string str1 = "GeeksforGeeks";
    leftrotate(str1, 2);
    cout << str1 << endl;

    string str2 = "GeeksforGeeks";
    rightrotate(str2, 2);
    cout << str2 << endl;
    return 0;
}

Output: 

Left rotation:  eksforGeeksGe
Right rotation:  ksGeeksforGee                 

Time Complexity: O(N), as we are using a loop to traverse N times so it will cost us O(N) time 
Auxiliary Space: O(1), as we are not using any extra space.

Method 2:

We can use extended string which is double in size of normal string to rotate string. For left rotation, access the extended string from index n to the index len(string) + n. For right rotation, rotate the string left with size-d places.

Approach:

The approach is

// Left rotate string s by d 
leftRotate(s, n)
 temp = s + s; // extended string 
 l1  = s.length // length of string 
 return temp[n : l1+n] //return rotated string.
 
// Right rotate string s by n
rightRotate(s, n)
 // We can also call above reverse steps
 // with x = s.length - n.
 leftRotate(s, x-n)

Below is implementation of above approach

// C++ program for Left Rotation and Right
// Rotation of a String
#include <bits/stdc++.h>
using namespace std;

// Rotating the string using extended string
string leftrotate(string str1, int n)
{

    // creating extended string and index for new rotated
    // string
    string temp = str1 + str1;
    int l1 = str1.size();

    string Lfirst = temp.substr(n, l1);

    //     now returning string
    return Lfirst;
}
// Rotating the string using extended string
string rightrotate(string str1, int n)
{

    return leftrotate(str1, str1.size() - n);
}

// Driver code
int main()
{
    string str1 = leftrotate("GeeksforGeeks", 2);
    cout << str1 << endl;

    string str2 = rightrotate("GeeksforGeeks", 2);
    cout << str2 << endl;
    return 0;
}

// This code is contributed by Susobhan Akhuli

eksforGeeksGe
ksGeeksforGee

Time Complexity: O(N), where N is the size of the given string.
Auxiliary Space: O(N)

Method 3:

This approach defines two functions for left and right rotation of a string using the rotate() function provided by the STL (Standard Template Library) in C++. The left_rotate_string() function rotates the string s by d positions to the left, while the right_rotate_string() function rotates the string s by d positions to the right. Both functions return the rotated string.

Approach:

The approach is

1. Define two functions: left_rotate_string() and right_rotate_string().
2. In left_rotate_string(), perform a left rotation on the string s by d elements using the rotate() function.
3. In right_rotate_string(), perform a right rotation on the string s by d elements using the rotate() function.
4. Return the rotated string.

Below is the implementation of the above approach:

// CPP program for Left Rotation and Right
// Rotation of a String
#include <algorithm>
#include <iostream>

using namespace std;

// Define the left_rotate_string function
string left_rotate_string(string s, int d)
{
    // Perform a left rotation on the string by d elements
    rotate(s.begin(), s.begin() + d, s.end());

    return s;
}

// Define the right_rotate_string function
string right_rotate_string(string s, int d)
{
    // Perform a right rotation on the string by d elements
    rotate(s.rbegin(), s.rbegin() + d, s.rend());

    return s;
}

int main()
{
    string s = "GeeksforGeeks";
    int d = 2;
    cout << "Left Rotation: " << left_rotate_string(s, d)
         << endl;
    cout << "Right Rotation: " << right_rotate_string(s, d)
         << endl;

    s = "qwertyu";
    d = 2;
    cout << "Left Rotation: " << left_rotate_string(s, d)
         << endl;
    cout << "Right Rotation: " << right_rotate_string(s, d)
         << endl;

    return 0;
}

// This code is contributed by Susobhan Akhuli

Left Rotation: eksforGeeksGe
Right Rotation: ksGeeksforGee
Left Rotation: ertyuqw
Right Rotation: yuqwert

Time complexity: O(n), where n is the length of the input string s. This is because the rotation operation requires visiting every character in the string exactly once.
Auxiliary Space: O(n)

Please refer complete article on Left Rotation and Right Rotation of a String for more details!