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

Recall & Review

beginner

What does the push operation do in a stack?

It adds a new item to the top of the stack.

beginner

In a stack, where is the new element placed during a push operation?

At the top of the stack, above all existing elements.

intermediate

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

It causes a stack overflow error because there is no space to add the new element.

intermediate

Explain the time complexity of the push operation on a stack.

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

beginner

Show the result of pushing elements 1, then 2, then 3 onto an empty stack.

The stack will look like: 3 -> 2 -> 1 -> null (3 is at the top).

What is the position of the new element after a push operation on a stack?

A Middle of the stack

B Bottom of the stack

C Top of the stack

D Random position

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

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

A O(n)

B O(log n)

C O(n^2)

D O(1)

Push operation takes constant time O(1) because it adds the element directly on top.

What error occurs if you push onto a full stack?

A Stack overflow

B Null pointer exception

C Stack underflow

D Index out of bounds

Pushing onto a full stack causes a stack overflow error.

If you push 5, then 10, then 15 onto an empty stack, what is the top element?

A 5

B 15

C 10

D Stack is empty

The last pushed element (15) is at the top of the stack.

Which data structure principle does push operation follow?

A Last In First Out (LIFO)

B First In First Out (FIFO)

C Random Access

D Priority Queue

Stack follows Last In First Out (LIFO) principle, so push adds to the top.

Describe the push operation on a stack and what happens to the stack after pushing an element.

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

Explain what happens if you try to push an element onto a stack that is already full.

Consider the stack's capacity and what pushing means in that context.

Practice

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

easy

A. Returns the bottom element of the stack

B. Removes the top element from the stack

C. Adds an element to the top of the stack

D. Checks if the stack is empty

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.

3. Final Answer:

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

4. Quick Check:

   Push = Add top [OK]

Hint: Push means add on top of stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Assuming push accesses bottom element

2. Which of the following is the correct syntax to push an element 5 onto a stack named s in Go?

easy

A. s.push(5)

B. s.Push(5)

C. push(s, 5)

D. s = append(s, 5)

Solution

1. Step 1: Understand Go slice as stack

   In Go, stacks are often implemented using slices. To push, append adds element at the end.

2. Step 2: Identify correct syntax

   Using append(s, 5) adds 5 to the slice s, simulating push.

3. Final Answer:

   s = append(s, 5) -> Option D

4. Quick Check:

   Go push = append slice [OK]

Hint: Use append to push in Go slices [OK]

Common Mistakes:

• Using lowercase push method which doesn't exist
• Calling push as a function without method
• Ignoring that slices need reassignment after append

3. What will be the output after executing this Go code?

stack := []int{1, 2, 3}
stack = append(stack, 4)
fmt.Println(stack)

medium

A. [1 2 3 4]

B. [4 1 2 3]

C. [1 2 3]

D. runtime error

Solution

1. Step 1: Initial stack state

   Stack slice starts as [1 2 3].

2. Step 2: Append 4 to stack

   Append adds 4 at the end, so stack becomes [1 2 3 4].

3. Final Answer:

   [1 2 3 4] -> Option A

4. Quick Check:

   Append adds at end [OK]

Hint: Append adds element at slice end [OK]

Common Mistakes:

• Expecting append to add at start
• Confusing append with insert
• Thinking append causes error without reassignment

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

var stack []int
stack.push(10)

medium

A. Stack is not initialized

B. push method does not exist for slices

C. Missing import for stack package

D. Cannot push integer 10

Solution

1. Step 1: Check stack type and methods

   Stack is a slice of int. Slices do not have a push method in Go.

2. Step 2: Identify correct push approach

   To push, use append function instead of push method.

3. Final Answer:

   push method does not exist for slices -> Option B

4. Quick Check:

   Slices lack push method [OK]

Hint: Slices use append, no push method [OK]

Common Mistakes:

• Assuming push is built-in method for slices
• Thinking stack must be initialized first
• Believing import fixes method absence

5. Given a stack implemented as a slice in Go, how would you push multiple elements 7, 8, 9 in one operation?

hard

A. stack = append(stack, []int{7, 8, 9}...)

B. stack.push(7); stack.push(8); stack.push(9)

C. append(stack, 7, 8, 9)

D. stack = append(stack, []int{7, 8, 9})

Solution

1. Step 1: Understand appending multiple elements

   To add multiple elements from a slice, use the ... operator to unpack them.

2. Step 2: Identify correct append syntax

   Using append(stack, []int{7, 8, 9}...) adds all three elements individually.

3. Final Answer:

   stack = append(stack, []int{7, 8, 9}...) -> Option A

4. Quick Check:

   Use ... to unpack slice in append [OK]

Hint: Use ... to append multiple elements at once [OK]

Common Mistakes:

• Forgetting to assign the result of append back to stack
• Using non-existent push method multiple times
• Appending slice without unpacking elements