Java Program to Reverse a String using Stack

Difficulty Level : Easy
Last Updated : 21 Oct, 2020

The Stack is a linear data structure that follows the LIFO(Last In First Out) principle, i.e, the element inserted at the last is the element to come out first.

Approach:

Push the character one by one into the Stack of datatype character.
Pop the character one by one from the Stack until the stack becomes empty.
Add a popped element to the character array.
Convert character array to string.
Return reversed string.

Below is the implementation of the above approach.

Java

// Java Program to Reverse a String using Stack
  
import java.io.*;
import java.util.*;
  
class GFG {
  
    public static String ReverseString(String str)
    {
        char[] reverseString = new char[str.length()];
        // Declare a stack of type Character
        Stack<Character> stack = new Stack<Character>();
  
        // Traverse the String and push the character one by
        // one into the Stack
        for (int i = 0; i < str.length(); i++) {
            // push the character into the Stack
            stack.push(str.charAt(i));
        }
  
        // Now Pop the Characters from the stack until it
        // becomes empty
  
        int i = 0;
        while (!stack.isEmpty()) { // popping element until
                                   // stack become empty
            // get the character from the top of the stack
            reverseString[i++] = stack.pop();
        }
        // return string object
        return new String(reverseString);
    }
  
    // Driver code
    public static void main(String[] args)
    {
        String str1 = "GeeksForGeeks";
        
        // call the function
        System.out.println(str1 + " <- Reverse -> "
                           + ReverseString(str1));
        
        String str2 = "Hello World";
        
        // call the function
        System.out.println(str2 + " <- Reverse -> "
                           + ReverseString(str2));
    }
}

Output:

GeeksForGeeks <- Reverse -> skeeGroFskeeG
Hello World <- Reverse -> dlroW olleH

Time Complexity: O(n), where n is a number of characters in the stack.

Auxiliary Space: O(n) for the stack.

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