JavaScript Program for Left Rotation and Right Rotation of a String

To implement left rotation and right rotation of a string, we can use various approaches. Left rotation means moving characters counter-clockwise by a given number of positions, while right rotation means moving characters clockwise by a given number of positions.

In this article we have a string and a value of k by which we will rotate the string. Our task is to write a JavaScript program for left rotation and right rotation of a string.

Example

Input:
String str = "apple";
k = 3

Output:
Left Rotation: "leapp"
Right Rotation: "pleap"

Approaches for Left and Right Rotation of String

Here are the approaches for left rotation and right rotation of a string in JavaScript which we will discuss with stepwise explanations and complete example codes.

• Using String Slicing
• Using String Concatenation
• Using Recursive Rotation

Using String Slicing

For left rotation and right rotation of string, we extract a part of the string and concatenate it with the remaining string by the value of k.

• First we get the given string in a variable str and store the number of rotations required in another variable k.
• We create two functions, left_rotation() for the left rotation and right_rotation() for the right rotation of the string. Both functions accept two arguments, str and k.
• In the left rotation function, we get two substrings using slice() method and switch their places.
• In the right rotation function, we get two substrings and switch their positions for clockwise movement.

Example

var str = "apple";
var k = 3;
console.log("The given string is: " + str);

function left_rotation(str, k) {
    k = k % str.length;
    var new_str = str.slice(k) + str.slice(0, k);
    console.log("String after " + k + " left rotation is: " + new_str);
}

function right_rotation(str, k) {
    k = k % str.length;
    var new_str = str.slice(str.length - k) + str.slice(0, str.length - k);
    console.log("String after " + k + " right rotation is: " + new_str);
}

left_rotation(str, k);
right_rotation(str, k);

The given string is: apple
String after 3 left rotation is: leapp
String after 3 right rotation is: pleap

Using String Concatenation

In this approach for left rotation and right rotation of string we concatenate the string with itself to create a doubled string, then extract the required substring.

• First we get the given string in a variable str and store the number of rotations required in another variable k.
• We create two functions, leftRotate() for the left rotation and rightRotate() for the right rotation of the string.
• Inside leftRotate function, we use modulo operator to ensure "k" does not exceed the string length.
• Then concatenate the string with itself and store it in variable doubled.
• The substr() function gets the string from "k" to string length for left rotation.
• For right rotation, we extract substring from position (str.length - k).

Example

var str = "apple";
var k = 3;
console.log("The given string is: " + str);

function leftRotate(str, k) {
    k = k % str.length; 
    const doubled = str + str;
    return doubled.substr(k, str.length);
}

function rightRotate(str, k) {
    k = k % str.length; 
    const doubled = str + str;
    return doubled.substr(str.length - k, str.length);
}

console.log("String after " + k + " left rotation is: " + leftRotate(str, k)); 
console.log("String after " + k + " right rotation is: " + rightRotate(str, k));

The given string is: apple
String after 3 left rotation is: leapp
String after 3 right rotation is: pleap

Using Recursive Rotation

In this approach we use recursive rotation where we call both functions until we achieve the desired result by rotating one character at a time.

• We create two functions, leftRotate() for the left rotation and rightRotate() for the right rotation.
• Inside the leftRotate() function we set the terminating condition when k==0.
• When it is not the terminating condition, we move the first element of string to end and concatenate with sliced string.
• The function calls itself recursively until k reaches 0.
• Similarly, we repeat the process for right rotation in rightRotate() function.

Example

var str = "apple";
var k = 3;
console.log("The given string is: " + str);

function leftRotate(str, k) {
    if (k === 0) return str;
    var new_str = leftRotate(str.slice(1) + str[0], k - 1);
    return new_str;
}

function rightRotate(str, k) {
    if (k === 0) return str;
    var new_str = rightRotate(str[str.length - 1] + str.slice(0, -1), k - 1);
    return new_str;
}

console.log("String after " + k + " left rotation is: " + leftRotate(str, k)); 
console.log("String after " + k + " right rotation is: " + rightRotate(str, k));

The given string is: apple
String after 3 left rotation is: leapp
String after 3 right rotation is: pleap

Complexity Comparison

Here is a comparison of time and space complexity of all the above approaches.

Conclusion

We have implemented JavaScript programs for left rotation and right rotation of strings using three different approaches: string slicing, string concatenation, and recursive rotation. String slicing is the most efficient approach for practical use cases.