Python3 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 : 

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:

eksforGeeksGe
ksGeeksforGee

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

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