Java: Merge two stacks into one

Last update on May 09 2025 12:50:16 (UTC/GMT +8 hours)

18. Merge two stacks into one.

Write a Java program to merge two stacks into one.

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 merge to stacks
  public static Stack mergeStacks(Stack stack1, Stack stack2) {
    int size1 = stack1.top + 1;
    int size2 = stack2.top + 1;
    Stack mergedStack = new Stack(size1 + size2);

    // Push all elements of stack1 onto the merged stack
    for (int i = 0; i < size1; i++) {
      mergedStack.push(stack1.pop());
    }

    // Push all elements of stack2 onto the merged stack
    for (int i = 0; i < size2; i++) {
      mergedStack.push(stack2.pop());
    }

    return mergedStack;
  }

  // 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) {
    System.out.println("Initialize a stack:");
    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(4);
    stack2.push(5);
    stack2.push(6);
    System.out.println("\nStack-2");
    stack2.display();
    System.out.println("\nMerge said two stacks:");
    Stack mergedStack = mergeStacks(stack1, stack2);
    mergedStack.display();

  }
}

Sample Output:

Stack-1
Stack elements: 3 2 1

Stack-2
Stack elements: 6 5 4

Merge said two stacks:
Stack elements: 4 5 6 1 2 3

Flowchart:

Flowchart: Java Exercises: Merge two stacks into one

Flowchart: Java Exercises: Merge two stacks into one.

For more Practice: Solve these Related Problems:

Write a Java program to merge two stacks by alternating elements from each stack using recursion.
Write a Java program to merge two sorted stacks into one sorted stack using an auxiliary stack.
Write a Java program to combine two stacks into one and remove duplicates during the merge process.
Write a Java program to merge two stacks and then reverse the merged stack to preserve the original order.

Go to:

PREV : Move the nth element from the top to the top.
NEXT : Check if one stack is a subset of another stack.

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Java Code Editor:

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