Stack Concept and LIFO Principle in DSA Go - Execution Trace

Concept Flow - Stack Concept and LIFO Principle

Start with empty stack

↓

Push element

→Add element on top

↓

Stack grows upward

↓

Pop element

→Remove top element

↓

Stack shrinks downward

↓

Repeat push/pop as needed

↓

Last In First Out (LIFO) principle applies

Stack adds elements on top and removes from top, following Last In First Out order.

Execution Sample

DSA Go

stack := []int{}
stack = append(stack, 10) // push 10
stack = append(stack, 20) // push 20
stack = stack[:len(stack)-1] // pop top
stack = append(stack, 30) // push 30

This code pushes 10, then 20, pops the top (20), then pushes 30 on the stack.

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Start with empty stack	[]	head = nil	null
2	Push 10	[10]	head points to 10	10 -> null
3	Push 20	[10, 20]	head points to 20	20 -> 10 -> null
4	Pop top (20)	[10]	head points to 10	10 -> null
5	Push 30	[10, 30]	head points to 30	30 -> 10 -> null

💡 No more operations, stack ends with top element 30.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
stack	[]	[10]	[10, 20]	[10]	[10, 30]
head (top)	nil	10	20	10	30
size	0	1	2	1	2

Key Moments - 3 Insights

Why does popping remove the last pushed element, not the first?

Because the stack follows LIFO, the last pushed element is always on top and removed first, as shown in step 4 where 20 is popped after pushing 10 and 20.

What happens if we pop from an empty stack?

Popping from an empty stack is invalid and would cause an error or no operation, but here the stack is never empty when popping, as seen in step 4.

Why does pushing add elements on top, not at the bottom?

Pushing adds elements on top to maintain LIFO order, so new elements are always accessible first, as shown in steps 2, 3, and 5 where new elements become the top.

Visual Quiz - 3 Questions

Look at the execution table, what is the stack state after step 3?

A [10, 20]

B [10]

C [20]

D []

💡 Hint

Check the 'Nodes in Stack' column at step 3 in the execution_table.

At which step does the stack size reduce?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'size' variable in variable_tracker and see when it decreases.

If we push 40 after step 5, what will be the new top element?

A 10

B 40

C 30

D 20

💡 Hint

Pushing adds element on top, so the new element becomes the top.

Concept Snapshot

Stack stores elements in a Last In First Out (LIFO) order.
Push adds an element on top.
Pop removes the top element.
Top always points to the last pushed element.
Stack size changes with push/pop.
Empty stack has no elements to pop.

Full Transcript

A stack is a data structure where elements are added and removed from the top only. This follows the Last In First Out principle, meaning the last element added is the first to be removed. We start with an empty stack. When we push 10, it becomes the top. Pushing 20 puts it on top of 10. Popping removes 20, leaving 10 on top. Pushing 30 adds it on top of 10. The stack grows and shrinks only at the top. This behavior is shown step-by-step in the execution table and variable tracker. Understanding this helps avoid confusion about which element is removed or added next.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the last element added is the first to be removed

B. Last In, First Out - the first element added is the first to be removed

C. First In, First Out - the first element added is the first to be removed

D. First In, Last Out - the first element added is the last to be removed

Solution

1. Step 1: Understand LIFO meaning

   LIFO stands for Last In, First Out, meaning the last item added to the stack is the first one removed.

2. Step 2: Match with stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

   Last In, First Out - the last element added is the first to be removed -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing up stack with queue behavior

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

easy

A. stack.push(5)

B. stack = append(stack, 5)

C. stack.add(5)

D. stack.insert(5)

Solution

1. Step 1: Recall Go slice push syntax

   In Go, stacks are often slices; to push, use append(slice, value).

2. Step 2: Check options for correct syntax

   Only stack = append(stack, 5) correctly adds 5 to the stack slice.

3. Final Answer:

   stack = append(stack, 5) -> Option B

4. Quick Check:

   Go stack push uses append [OK]

Hint: Use append to add elements to Go slices [OK]

Common Mistakes:

• Using non-existent push method on slice
• Using add or insert which are invalid for slices
• Forgetting to assign append result back

3. What will be the output of the following Go code snippet?

stack := []int{}
stack = append(stack, 10)
stack = append(stack, 20)
stack = append(stack, 30)
last := stack[len(stack)-1]
stack = stack[:len(stack)-1]
fmt.Println(last)

medium

A. 30

B. 20

C. 10

D. Error

Solution

1. Step 1: Trace stack push operations

   Elements 10, 20, 30 are pushed in order; stack is [10, 20, 30].

2. Step 2: Pop last element

   last = stack[len(stack)-1] gets 30; then stack is sliced to remove last element.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed element popped = 30 [OK]

Hint: Last appended element is popped first [OK]

Common Mistakes:

• Printing first element instead of last
• Forgetting to slice stack after pop
• Confusing index for pop operation

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

stack := []int{1, 2, 3}
last := stack[len(stack)]
stack = stack[:len(stack)-1]
fmt.Println(last)

medium

A. Cannot print variable last

B. Syntax error in slice operation

C. Index out of range error due to accessing stack[len(stack)]

D. No error, code runs correctly

Solution

1. Step 1: Check index used for popping

   Indexing with stack[len(stack)] is invalid because slice indices go from 0 to len(stack)-1.

2. Step 2: Identify error type

   This causes an index out of range runtime error.

3. Final Answer:

   Index out of range error due to accessing stack[len(stack)] -> Option C

4. Quick Check:

   Index must be len(stack)-1, not len(stack) [OK]

Hint: Use len(stack)-1 for last index [OK]

Common Mistakes:

• Using len(stack) as index
• Ignoring zero-based indexing
• Assuming code runs without error

5. Given a stack implemented as a slice in Go, how would you reverse the order of elements using only stack operations (push and pop)?

hard

A. Pop elements and discard them to reverse the stack

B. Push elements onto the same stack again without popping

C. Use a queue instead of a stack to reverse elements

D. Pop all elements into a temporary slice, then push them back in order

Solution

1. Step 1: Pop all elements from original stack

   Pop elements one by one and store them in a temporary slice to hold reversed order.

2. Step 2: Push elements back to original stack

   Push elements from temporary slice back to stack; this reverses the order due to LIFO.

3. Final Answer:

   Pop all elements into a temporary slice, then push them back in order -> Option D

4. Quick Check:

   Reversing stack needs pop then push back [OK]

Hint: Pop all, then push back reverses stack [OK]

Common Mistakes:

• Pushing without popping does not reverse
• Discarding elements loses data
• Using queue is not stack operation