Java: Elements from two stacks without duplicates

Last update on May 09 2025 12:50:13 (UTC/GMT +8 hours)

23. Create a new stack without duplicates from two stacks.

Write a Java program that implements a stack and creates a new stack that contains all elements from two stacks without duplicates.

Sample Solution:

Java Code:

import java.util.Scanner;

public class Stack {
  private int[] arr;
  private int top;

  // Constructor to initialize the stack
  public Stack(int size) {
    arr = new int[size];
    top = -1;
  }

  // Method to push an element onto the stack
  public void push(int num) {
    if (top == arr.length - 1) {
      System.out.println("Stack is full");
    } else {
      top++;
      arr[top] = num;
    }
  }

  // Method to pop an element from the stack
  public int pop() {
    if (top == -1) {
      System.out.println("Stack Underflow");
      return -1;
    } else {
      int poppedElement = arr[top];
      top--;
      return poppedElement;
    }
  }

  // Method to get the top element of the stack
  public int peek() {
    if (top == -1) {
      System.out.println("Stack is empty");
      return -1;
    } else {
      return arr[top];
    }
  }

  // Method to check if the stack is empty
  public boolean isEmpty() {
    return top == -1;
  }

  // Method to get the size of the stack
  public int getSize() {
    return top + 1;
  }

  public static Stack findUniqueElements(Stack stack1, Stack stack2) {
    Stack resultStack = new Stack(stack1.arr.length + stack2.arr.length);

    // Add all elements from stack1 to resultStack
    for (int i = 0; i <= stack1.top; i++) {
      int element = stack1.arr[i];
      if (!resultStack.contains(element)) {
        resultStack.push(element);
      }
    }

    // Add all elements from stack2 to resultStack
    for (int i = 0; i <= stack2.top; i++) {
      int element = stack2.arr[i];
      if (!resultStack.contains(element)) {
        resultStack.push(element);
      }
    }

    return resultStack;
  }

  // Helper method to check if a stack contains an element
  private boolean contains(int element) {
    for (int i = 0; i <= top; i++) {
      if (arr[i] == element) {
        return true;
      }
    }
    return false;
  }

  // Method to display the elements of the stack
  public void display() {
    if (top == -1) {
      System.out.println("Stack is empty!");
    } else {
      System.out.print("Stack elements: ");
      for (int i = top; i >= 0; i--) {
        System.out.print(arr[i] + " ");
      }
      System.out.println();
    }
  }

  public static void main(String[] args) {
    Stack result = new Stack(5);
    Stack stack1 = new Stack(5);
    stack1.push(1);
    stack1.push(2);
    stack1.push(3);
    System.out.println("\nStack-1");
    stack1.display();
    Stack stack2 = new Stack(5);
    stack2.push(3);
    stack2.push(5);
    stack2.push(6);
    System.out.println("\nStack-2");
    stack2.display();
    Stack stack3 = new Stack(5);
    stack3.push(1);
    stack3.push(2);
    stack3.push(4);
    System.out.println("\nStack-3");
    stack3.display();
    System.out.println("\nNew stack that contains all elements from two stacks (stack1, stack2) without duplicates:");
    result = findUniqueElements(stack1, stack2);
    result.display();
    System.out.println("\nNew stack that contains all elements from two stacks (stack1, stack3) without duplicates:");
    result = findUniqueElements(stack1, stack3);
    result.display();
    System.out.println("\nNew stack that contains all elements from two stacks (stack3, stack2) without duplicates:");
    result = findUniqueElements(stack3, stack2);
    result.display();
  }
}

Sample Output:

Stack-1
Stack elements: 3 2 1

Stack-2
Stack elements: 6 5 3

Stack-3
Stack elements: 4 2 1

New stack that contains all elements from two stacks (stack1, stack2) without duplicates:
Stack elements: 6 5 3 2 1

New stack that contains all elements from two stacks (stack1, stack3) without duplicates:
Stack elements: 4 3 2 1

New stack that contains all elements from two stacks (stack3, stack2) without duplicates:
Stack elements: 6 5 3 4 2 1

Flowchart:

Flowchart: Java Exercises: Elements from two stacks without duplicates.

For more Practice: Solve these Related Problems:

Write a Java program to merge two stacks and then remove duplicates using a LinkedHashSet to preserve order.
Write a Java program to combine two stacks into a new stack that contains only unique elements using streams.
Write a Java program to merge two stacks iteratively and check for duplicates before pushing each element.
Write a Java program to implement a method that merges two stacks and uses recursion to filter out duplicate elements.

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