Reverse a String using Stack in Java

This post will discuss how to reverse a string using Stack in Java.

The following example demonstrates how to reverse a string with a Stack data structure in Java. Following are the complete steps:

Create an empty stack of characters.
Convert given string into character array using String.toCharArray() method and push each character of it into the stack.
Remove characters from the stack until it becomes empty and assign them back to the character array. As stack follows FILO order, characters will be inserted in the reverse order.
Finally, convert the character array into string using String.copyValueOf(char[]) and return the formed string.

The following program demonstrates it:

import java.util.Stack;

class Main

{

// Method to reverse a string in Java using a stack and character array

public static String reverse(String str)

{

// base case: if the string is null or empty

if (str == null || str.equals("")) {

return str;

}

// create an empty stack of characters

Stack<Character> stack = new Stack<Character>();

// push every character of the given string into the stack

char[] ch = str.toCharArray();

for (int i = 0; i < str.length(); i++) {

stack.push(ch[i]);

}

// start from index 0

int k = 0;

// pop characters from the stack until it is empty

while (!stack.isEmpty())

{

// assign each popped character back to the character array

ch[k++] = stack.pop();

}

// convert the character array into a string and return it

return String.copyValueOf(ch);

}

public static void main(String[] args)

{

String str = "Reverse me!";

str = reverse(str); // string is immutable

System.out.println("The reversed string is " + str);

}

Download Run Code

Output:

The reversed string is !em esreveR

We can also use StringBuilder instead of a character array, as shown below:

import java.util.Stack;

class Main

{

// Method to reverse a string in Java using a stack and StringBuilder

public static String reverse(String str)

{

// base case: if the string is null or empty

if (str == null || str.equals("")) {

return str;

}

// create an empty stack of characters

Stack<Character> stack = new Stack<Character>();

// push every character of the given string into the stack

for (int i = 0; i < str.length(); i++) {

stack.push(str.charAt(i));

}

// pop characters from the stack and append them into StringBuilder

StringBuilder sb = new StringBuilder();

while (!stack.isEmpty()) {

sb.append(stack.pop());

}

// convert `StringBuilder` to string and return

return sb.toString();

}