Stack Implementation Using Array in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack in data structures?

A stack is a collection that follows Last In, First Out (LIFO) order, where the last element added is the first one to be removed.

beginner

What does the 'push' operation do in a stack implemented using an array?

The 'push' operation adds a new element to the top of the stack if there is space available in the array.

intermediate

What happens when you try to push an element into a full stack implemented with an array?

It causes a stack overflow error because the array has no more space to hold new elements.

beginner

Explain the 'pop' operation in a stack implemented using an array.

The 'pop' operation removes and returns the top element of the stack if the stack is not empty. If empty, it causes a stack underflow error.

beginner

How do you check if a stack implemented using an array is empty?

By checking if the top index is -1, which means no elements are in the stack.

What is the initial value of the 'top' variable in an array-based stack?

A -1

B 0

C 1

D Array length

The 'top' starts at -1 to indicate the stack is empty.

What does the 'pop' operation return when the stack is empty?

A Zero

B The last element

C Null

D Stack underflow error or indication

Popping from an empty stack causes underflow, so it returns an error or indication.

Which condition checks for stack overflow in an array-based stack?

A top == -1

B top == 0

C top == array.length - 1

D array.length == 0

When 'top' reaches the last index (array.length - 1), the stack is full.

What is the time complexity of push and pop operations in an array-based stack?

A O(n)

B O(1)

C O(log n)

D O(n^2)

Both push and pop operations take constant time O(1).

What happens if you try to pop an element from an empty stack?

A Stack underflow

B Stack overflow

C Push operation

D No change

Popping from an empty stack causes stack underflow.

Describe how to implement a stack using an array, including how to handle push and pop operations.

Think about how the 'top' variable changes with each operation.

Explain what stack overflow and stack underflow mean in the context of an array-based stack.

Consider the limits of the array and the top index.

Practice

1. What is the main purpose of using an array in stack implementation?

easy

A. To allow random access to elements anywhere in the stack

B. To store elements in a fixed-size linear structure

C. To automatically resize the stack when full

D. To store elements in a linked manner with pointers

Solution

1. Step 1: Understand stack storage using array

   An array provides a fixed-size linear block of memory to store stack elements sequentially.

2. Step 2: Compare with other storage methods

   Unlike linked lists, arrays do not use pointers and have fixed size, so they store elements linearly.

3. Final Answer:

   To store elements in a fixed-size linear structure -> Option B

4. Quick Check:

   Array = fixed-size linear storage [OK]

Hint: Stack array stores elements linearly with fixed size [OK]

Common Mistakes:

• Thinking array allows automatic resizing
• Confusing array with linked list storage
• Assuming random access is main stack feature

2. Which of the following is the correct way to declare a stack array of size 10 in Java?

easy

A. int[] stack = new int[10];

B. int stack = new int[10];

C. int stack[] = int[10];

D. int stack = int[10];

Solution

1. Step 1: Recall Java array declaration syntax

   In Java, arrays are declared with type[], variable = new type[size];

2. Step 2: Match correct syntax

   int[] stack = new int[10]; matches this syntax: int[] stack = new int[10]; others have syntax errors.

3. Final Answer:

   int[] stack = new int[10]; -> Option A

4. Quick Check:

   Java array declaration = int[] name = new int[size]; [OK]

Hint: Use 'type[] name = new type[size];' for arrays in Java [OK]

Common Mistakes:

• Missing 'new' keyword
• Wrong placement of brackets
• Assigning array without 'new'

3. What will be the output after the following operations on a stack of size 3?

Stack s = new Stack(3);
s.push(10);
s.push(20);
s.pop();
s.push(30);
s.push(40);

Assuming push prints "Pushed: value" and pop prints "Popped: value".

medium

A. "Pushed: 10\nPushed: 20\nPopped: 20\nStack Overflow\nPushed: 40"

B. "Pushed: 10\nPushed: 20\nPopped: 20\nPushed: 30"

C. "Pushed: 10\nPushed: 20\nPopped: 20\nPushed: 30\nStack Overflow"

D. "Pushed: 10\nPushed: 20\nPopped: 20\nPushed: 30\nPushed: 40"

Solution

1. Step 1: Trace push and pop operations on stack size 3

   push(10) prints "Pushed: 10" stack:[10]; push(20) "Pushed: 20" [10,20]; pop() "Popped: 20" [10]; push(30) "Pushed: 30" [10,30]; push(40) "Pushed: 40" [10,30,40] (capacity 3, net 3 elements).

2. Step 2: Identify outputs printed

   All pushes and pop print their messages; no overflow since 3 <= capacity 3.

3. Final Answer:

   "Pushed: 10\nPushed: 20\nPopped: 20\nPushed: 30\nPushed: 40" -> Option D

4. Quick Check:

   Net 3 elements <= size 3 = no overflow [OK]

Hint: Net pushes (4 push - 1 pop = 3) <= size 3: no overflow [OK]

Common Mistakes:

• Thinking stack doesn't reuse space after pop
• Miscalculating net elements (3 <= 3)
• Expecting overflow on 3rd net push

4. Identify the error in the following stack push method and select the correct fix:

public void push(int x) {
  if (top == size) {
    System.out.println("Stack Overflow");
  } else {
    stack[top] = x;
    top++;
  }
}

medium

A. Change condition to if (top >= size) to correctly detect overflow

B. Change condition to if (top == size - 1) to avoid overflow

C. Initialize top to 1 instead of 0 to fix indexing

D. Replace top++ with ++top to fix increment order

Solution

1. Step 1: Analyze overflow condition

   Since array indices go from 0 to size-1, top == size means stack is full; condition should catch top >= size to prevent overflow.

2. Step 2: Check other options

   Changing to size - 1 is incorrect because top points to next free slot; initializing top to 1 or changing increment order is unrelated.

3. Final Answer:

   Change condition to if (top >= size) to correctly detect overflow -> Option A

4. Quick Check:

   Overflow check = top >= size [OK]

Hint: Overflow when top index reaches or exceeds size [OK]

Common Mistakes:

• Using top == size - 1 instead of top >= size
• Incorrect top initialization
• Misunderstanding increment order

5. Given a stack implemented using an array with size 5, which sequence of operations will result in the stack containing elements [5, 10, 15] from bottom to top?

Operations:
1) push(5)
2) push(10)
3) pop()
4) push(15)
5) push(10)

hard

A. Perform operations in order: 1, 2, 3, 4, 5

B. Perform operations in order: 1, 3, 2, 4, 5

C. Perform operations in order: 1, 2, 5, 3, 4

D. Perform operations in order: 1, 2, 4, 3, 5

Solution

1. Step 1: Trace operations in order 1,2,3,4,5

   push(5) -> [5]; push(10) -> [5,10]; pop() removes 10 -> [5]; push(15) -> [5,15]; push(10) -> [5,15,10]

2. Step 2: Check final stack order bottom to top

   Stack is [5,15,10], which is not [5,10,15]. So Perform operations in order: 1, 2, 3, 4, 5 is incorrect. Check other options similarly.

3. Step 3: Trace Perform operations in order: 1, 3, 2, 4, 5 (1,3,2,4,5)

   push(5) -> [5]; pop() removes 5 -> []; push(10) -> [10]; push(15) -> [10,15]; push(10) -> [10,15,10] not correct.

4. Step 4: Trace Perform operations in order: 1, 2, 4, 3, 5 (1,2,4,3,5)

   push(5) -> [5]; push(10) -> [5,10]; push(15) -> [5,10,15]; pop() removes 15 -> [5,10]; push(10) -> [5,10,10] not correct.

5. Step 5: Trace Perform operations in order: 1, 2, 5, 3, 4 (1,2,5,3,4)

   push(5) -> [5]; push(10) -> [5,10]; push(10) -> [5,10,10]; pop() removes 10 -> [5,10]; push(15) -> [5,10,15] correct final stack.

6. Final Answer:

   Perform operations in order: 1, 2, 5, 3, 4 -> Option C

7. Quick Check:

   Final stack = [5,10,15] after Perform operations in order: 1, 2, 5, 3, 4 [OK]

Hint: Simulate stack step-by-step to verify final order [OK]

Common Mistakes:

• Assuming order of operations does not affect stack
• Not simulating pop effects correctly
• Confusing push order with final stack order