Push Operation on Stack in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the push operation do in a stack?

The push operation adds a new element to the top of the stack.

beginner

In a stack, where is the new element placed when you perform a push?

The new element is placed at the top of the stack, above all existing elements.

intermediate

What happens if you try to push an element onto a full stack?

This situation is called stack overflow, and the push operation cannot add the new element.

beginner

Show the state of the stack after pushing elements 5, then 10, then 15.

Stack from top to bottom: 15 -> 10 -> 5 -> null

intermediate

What is the time complexity of the push operation on a stack?

The push operation runs in O(1) time because it adds an element at the top without traversing the stack.

What does the push operation do in a stack?

A Checks if the stack is empty

B Removes the top element

C Adds an element to the top

D Returns the bottom element

Push adds a new element to the top of the stack.

If a stack is full, what happens when you try to push a new element?

A Stack overflow occurs

B The bottom element is removed

C The stack automatically resizes

D Nothing happens, push succeeds

Pushing onto a full stack causes stack overflow, so the push fails.

After pushing 3 elements onto an empty stack, how many elements does it have?

A 0

B 1

C 2

D 3

Each push adds one element, so 3 pushes mean 3 elements.

What is the time complexity of the push operation on a stack?

A O(1)

B O(log n)

C O(n^2)

D O(n)

Push runs in constant time O(1) because it only adds an element at the top.

Where is the new element placed when you push it onto a stack?

A At the bottom

B At the top

C In the middle

D At a random position

Push always places the new element at the top of the stack.

Explain how the push operation works on a stack and what happens to the stack's state after pushing an element.

Think about where the new element goes and how the stack changes.

Describe what stack overflow means in the context of the push operation.

Consider what happens when there is no space left to add.

Practice

1. What does the push operation do in a stack?

easy

A. Adds an element to the top of the stack

B. Removes the top element from the stack

C. Checks if the stack is empty

D. Returns the bottom element of the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so new elements are added on top.

2. Step 2: Define push operation

   The push operation adds a new element to the top of the stack, increasing its size by one.

3. Final Answer:

   Adds an element to the top of the stack -> Option A

4. Quick Check:

   Push = Add top element [OK]

Hint: Push always adds element on top of stack [OK]

Common Mistakes:

• Confusing push with pop
• Thinking push removes element
• Assuming push adds at bottom

2. Which of the following is the correct syntax to push an integer 10 onto a stack named stack in Java?

easy

A. stack.add(10);

B. stack.push(10);

C. stack.insert(10);

D. stack.put(10);

Solution

1. Step 1: Recall Java Stack API

   Java's Stack class uses the method push() to add elements.

2. Step 2: Match correct method

   Among options, only push() is the correct method to add elements to a stack.

3. Final Answer:

   stack.push(10); -> Option B

4. Quick Check:

   Java stack push method = push() [OK]

Hint: Use push() method to add elements in Java stack [OK]

Common Mistakes:

• Using add() which is for lists
• Using insert() which doesn't exist
• Using put() which is for maps

3. What will be the output after executing the following Java code?

Stack<Integer> stack = new Stack<>();
stack.push(5);
stack.push(10);
stack.push(15);
System.out.println(stack);

medium

A. [5, 15, 10]

B. [15, 10, 5]

C. [10, 15, 5]

D. [5, 10, 15]

Solution

1. Step 1: Trace push operations

   Elements 5, 10, and 15 are pushed in order, so stack stores them as [5, 10, 15].

2. Step 2: Understand Stack's toString()

   Java's Stack prints elements from bottom to top in list format, so output is [5, 10, 15].

3. Final Answer:

   [5, 10, 15] -> Option D

4. Quick Check:

   Stack print shows bottom to top [OK]

Hint: Stack prints bottom to top in Java [OK]

Common Mistakes:

• Assuming print shows top element first
• Confusing stack with queue order
• Expecting reversed list output

4. Identify the error in the following code snippet that tries to push an element onto a stack:

Stack<String> stack = new Stack<>();
stack.push("Hello");
stack.push(100);

medium

A. Stack must be initialized with size

B. Stack class does not have push method

C. Cannot push integer 100 into a Stack of Strings

D. Missing semicolon after push statements

Solution

1. Step 1: Check stack type and pushed elements

   The stack is declared to hold Strings only, but 100 is an integer.

2. Step 2: Identify type mismatch error

   Pushing an integer into a String stack causes a compile-time type error.

3. Final Answer:

   Cannot push integer 100 into a Stack of Strings -> Option C

4. Quick Check:

   Type mismatch error = pushing wrong data type [OK]

Hint: Stack type must match pushed element type [OK]

Common Mistakes:

• Ignoring generic type mismatch
• Thinking push method missing
• Assuming size needed at init

5. Given an empty stack, which sequence of push operations will result in the stack containing elements in this order from bottom to top: 3 -> 7 -> 5 -> null?

hard

A. push(3), push(7), push(5)

B. push(5), push(7), push(3)

C. push(7), push(3), push(5)

D. push(5), push(3), push(7)

Solution

1. Step 1: Understand stack order after pushes

   Elements pushed first are at bottom, last pushed at top.

2. Step 2: Match push order to desired stack

   To get 3 at bottom, then 7, then 5 on top, push in order: 3, then 7, then 5.

3. Final Answer:

   push(3), push(7), push(5) -> Option A

4. Quick Check:

   Push order = bottom to top [OK]

Hint: Push elements in bottom-to-top order [OK]

Common Mistakes:

• Reversing push order
• Confusing push with pop order
• Assuming stack inserts at bottom