# Java Exercises: Swap the top two elements of a stack

## Java Stack: Exercise-14 with Solution

Write a Java program to swap the top two elements of a given stack.

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() {
}

public void swapTopTwo() {
if (top < 1) {
System.out.println("Stack has less than two elements");
} else {
int temp = arr[top];
arr[top] = arr[top - 1];
arr[top - 1] = temp;
}
}

// 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 stack = new Stack(5);
System.out.println("Input some elements on the stack:");
stack.push(1);
stack.push(2);
stack.push(3);
stack.push(4);
stack.push(5);
stack.display();
stack.swapTopTwo();
System.out.println("\nAfter swapping top two elements:");
stack.display();
System.out.println("\nAgain swapping top two elements:");
stack.swapTopTwo();
stack.display();
}
}
```
```

Sample Output:

```Initialize a stack:
Input some elements on the stack:
Stack elements: 5 4 3 2 1

After swapping top two elements:
Stack elements: 4 5 3 2 1

Again swapping top two elements:
Stack elements: 5 4 3 2 1
```

Flowchart:

Live Demo:

Java Code Editor:

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Java Stack Previous: Remove a specific element from a stack.
Java Stack Exercises Next: Nth element from the top of the stack.

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