Stack Concept and LIFO Principle in DSA Rust - Execution Trace

Concept Flow - Stack Concept and LIFO Principle

Start with empty stack

↓

Push element

→Add element on top

↓

Stack grows upwards

↓

Pop element

→Remove element from top

↓

Stack shrinks

↓

Repeat push/pop as needed

↓

Last In, First Out (LIFO) order

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

Execution Sample

DSA Rust

let mut stack = Vec::new();
stack.push(10);
stack.push(20);
let top = stack.pop();

Push 10 and 20 onto stack, then pop top element (20).

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Initialize empty stack	[]	head = None	null
2	Push 10	[10]	head -> 10	10 -> null
3	Push 20	[20, 10]	head -> 20	20 -> 10 -> null
4	Pop top element	[10]	head -> 10	10 -> null

💡 Pop returns 20 demonstrating LIFO; stack left with [10]. Further pop returns 10, then empty (returns None).

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4
stack	[]	[10]	[20, 10]	[10]
head	None	10	20	10
top (pop result)	None	None	None	Some(20)

Key Moments - 3 Insights

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

Because stack follows LIFO: last pushed (step 3) is first popped (step 4), as shown in execution_table rows 3 and 4.

What happens if we pop from an empty stack?

Pop returns None or error because stack is empty, no elements to remove.

Why does push add elements on top, not bottom?

Push adds on top to keep LIFO order; new element becomes head (step 2 and 3 show head pointer moving to new element).

Visual Quiz - 3 Questions

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

A [20, 10]

B [10]

C []

D [30, 20, 10]

💡 Hint

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

Look at the execution_table, after which step is the stack [10]?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look for 'Nodes in Stack' showing [10] in execution_table.

If we push 30 after step 4, what will be the new top element?

A 10

B 20

C 30

D None

💡 Hint

Pushing adds element on top; see pointer changes in execution_table steps 2 and 3.

Concept Snapshot

Stack stores elements in LIFO order.
Push adds element on top.
Pop removes element from top.
Last pushed is first popped.
Empty stack returns None on 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 (LIFO) principle. When we push elements, they go on top of the stack. When we pop, we remove the top element, which is the last one added. The example code pushes 10, then 20, then pops the top element which is 20. The execution table shows the stack growing and shrinking step by step, with the head pointer always pointing to the top element. If we pop from an empty stack, it returns None. This behavior ensures the last element added is always the first to be removed.

Practice

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

easy

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

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

C. Last In, First Out - the last 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 means the last item added to the stack is the first one to be taken out.

2. Step 2: Match with options

   Last In, First Out - the last element added is the first to be removed correctly states "Last In, First Out" which matches the LIFO principle.

3. Final Answer:

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

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 order of insertion and removal

2. Which of the following is the correct Rust syntax to push an element 10 onto a stack named stack?

easy

A. stack.push(10);

B. stack.add(10);

C. stack.insert(10);

D. stack.append(10);

Solution

1. Step 1: Recall Rust Vec stack push method

   In Rust, a stack is often implemented using a Vec, and pushing uses the push method.

2. Step 2: Check method names

   Only push is the correct method to add an element at the end of a Vec.

3. Final Answer:

   stack.push(10); -> Option A

4. Quick Check:

   Rust Vec push method = push() [OK]

Hint: Use push() to add elements to Rust Vec stack [OK]

Common Mistakes:

• Using add() which does not exist
• Using insert() without index
• Using append() which expects another collection

3. What will be the output of the following Rust code?

let mut stack = Vec::new();
stack.push(5);
stack.push(10);
let x = stack.pop();
println!("{:?}", x);

medium

A. Some(10)

B. 10

C. None

D. Some(5)

Solution

1. Step 1: Trace stack push operations

   Stack after pushes: [5, 10] (10 is last pushed)

2. Step 2: Trace pop operation

   Pop removes last element 10 and returns Some(10) because pop returns Option type.

3. Final Answer:

   Some(10) -> Option A

4. Quick Check:

   pop() returns Some(last element) = Some(10) [OK]

Hint: pop() returns Some(last pushed) or None if empty [OK]

Common Mistakes:

• Thinking pop returns raw value, not Option
• Confusing order of elements popped
• Expecting None instead of Some value

4. Identify the error in this Rust code snippet that tries to pop from an empty stack:

let mut stack: Vec<i32> = Vec::new();
let value = stack.pop().unwrap();
println!("{}", value);

medium

A. pop() method does not exist on Vec

B. Calling unwrap() on None causes panic

C. stack is not mutable

D. println! macro syntax is incorrect

Solution

1. Step 1: Understand pop() on empty Vec

   pop() returns None if Vec is empty.

2. Step 2: Effect of unwrap() on None

   Calling unwrap() on None causes a runtime panic (crash).

3. Final Answer:

   Calling unwrap() on None causes panic -> Option B

4. Quick Check:

   unwrap() on None = panic [OK]

Hint: Never unwrap pop() without checking if stack is empty [OK]

Common Mistakes:

• Assuming pop() always returns Some
• Ignoring mutability requirement
• Misreading println! syntax

5. Given a stack implemented as a Rust Vec, what will be the content of the stack after the following sequence of operations?

Operations:
1) push(1)
2) push(2)
3) pop()
4) push(3)
5) push(2)
6) pop()

hard

A. Stack after operations is [1, 2]

B. Stack after operations is [1, 2, 3]

C. Stack after operations is [1, 3, 2]

D. Stack after operations is [1, 3]

Solution

1. Step 1: Simulate each operation step-by-step

   Start: [] 1) push(1) -> [1] 2) push(2) -> [1, 2] 3) pop() removes 2 -> [1] 4) push(3) -> [1, 3] 5) push(2) -> [1, 3, 2] 6) pop() removes 2 -> [1, 3]

2. Step 2: Compare final stack with options

   Final stack is [1, 3], which matches Stack after operations is [1, 3].

3. Final Answer:

   Stack after operations is [1, 3] -> Option D

4. Quick Check:

   Trace pushes and pops carefully [OK]

Hint: Track pushes and pops stepwise to find final stack [OK]

Common Mistakes:

• Ignoring pop removes last pushed element
• Mixing order of push and pop
• Assuming pop removes first element