Min Stack Design in DSA Go - Build from Scratch

Min Stack Design

📖 Scenario: Imagine you are building a special stack for a game score tracker. You want to keep track of the scores, but also quickly know the lowest score at any time.

🎯 Goal: Build a stack that can push scores, pop scores, and always tell the minimum score in the stack instantly.

📋 What You'll Learn

Create a stack data structure using two slices: one for all scores and one for minimum scores.

Add a variable to track the current minimum score.

Implement push and pop operations that update the minimum score stack correctly.

Implement a function to get the current minimum score.

Print the minimum score after some operations.

💡 Why This Matters

🌍 Real World

Min stacks are useful in scenarios like tracking minimum prices, scores, or temperatures efficiently in real-time.

💼 Career

Understanding min stack design helps in coding interviews and building efficient data structures for performance-critical applications.

1

Create the initial stack structure

Create two slices called stack and minStack to hold integers. Initialize both as empty slices.

DSA Go

# Create two slices called stack and minStack to hold integers
# Your code here

Hint

Use varName := []int{} to create empty slices in Go.

2

Add a variable to track the current minimum

Add a variable called currentMin of type int and set it to a very large number like int(^uint(0) >> 1) to represent infinity.

DSA Go

package main

import "fmt"

func main() {
    stack := []int{}
    minStack := []int{}
    // Add a variable currentMin set to a very large int
    // Your code here
}

Hint

This expression gives the maximum int value in Go, useful as a starting minimum.

3

Implement push and pop logic with minimum tracking

Write a function called push that takes an integer val, appends it to stack, and updates minStack and currentMin if val is smaller or equal to currentMin. Also write a function called pop that removes the last element from stack and updates minStack and currentMin accordingly.

DSA Go

package main

import "fmt"

func main() {
    stack := []int{}
    minStack := []int{}
    currentMin := int(^uint(0) >> 1)

    // Write push function here
    // Your code here

    // Write pop function here
    // Your code here
}

Hint

Remember to update currentMin when pushing or popping values.

4

Print the minimum value after operations

Use the push function to add 5, 3, 7, and 2 to the stack. Then use the pop function once. Finally, print the current minimum value stored in currentMin using fmt.Println(currentMin).

DSA Go

package main

import "fmt"

func main() {
    stack := []int{}
    minStack := []int{}
    currentMin := int(^uint(0) >> 1)

    push := func(val int) {
        stack = append(stack, val)
        if val > 1)
            } else {
                currentMin = minStack[len(minStack)-1]
            }
        }
    }

    // Push 5, 3, 7, 2
    // Pop once
    // Print currentMin
    // Your code here
}

Hint

After popping the last pushed value (2), the minimum should be 3.

Practice

1. What is the main purpose of a Min Stack data structure?

easy

A. To retrieve the minimum element in constant time

B. To sort elements in ascending order

C. To store elements in a queue fashion

D. To reverse the order of elements

Solution

1. Step 1: Understand Min Stack functionality

   A Min Stack is designed to keep track of the minimum element efficiently.

2. Step 2: Identify the key feature

   It allows retrieving the minimum element in constant time, unlike a normal stack.

3. Final Answer:

   To retrieve the minimum element in constant time -> Option A

4. Quick Check:

   Min Stack = Fast minimum retrieval [OK]

Hint: Min Stack always tracks minimum for O(1) retrieval [OK]

Common Mistakes:

• Thinking it sorts elements
• Confusing with queue behavior
• Assuming it reverses elements

2. Which of the following Go code snippets correctly declares a Min Stack struct with two slices for values and minimums?

easy

A. type MinStack struct { values map[int]int; mins []int }

B. type MinStack struct { values []int; min int }

C. type MinStack struct { values []int; mins []int }

D. type MinStack struct { values []int; mins int }

Solution

1. Step 1: Check struct fields for Min Stack

   Min Stack needs slices to store all values and track minimums at each push.

2. Step 2: Validate field types

   Both values and mins should be slices of int for stack behavior.

3. Final Answer:

   type MinStack struct { values []int; mins []int } -> Option C

4. Quick Check:

   Two int slices = correct Min Stack struct [OK]

Hint: Min Stack uses two int slices for values and mins [OK]

Common Mistakes:

• Using map instead of slice for values
• Using int instead of slice for mins
• Mixing types between fields

3. Given the following Go code for a Min Stack, what will be printed after the operations?

ms := MinStack{}
ms.Push(3)
ms.Push(5)
ms.Push(2)
ms.Push(1)
ms.Pop()
fmt.Println(ms.GetMin())

medium

A. 2

B. 1

C. 3

D. 5

Solution

1. Step 1: Trace Push operations and minimums

   Push 3 (min=3), Push 5 (min=3), Push 2 (min=2), Push 1 (min=1).

2. Step 2: Pop removes last pushed (1), so min updates to previous min (2)

   After pop, minimum is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Pop last min 1, new min = 2 [OK]

Hint: Pop removes last min, so min updates to previous min [OK]

Common Mistakes:

• Assuming min stays 1 after pop
• Confusing push order
• Not updating min stack on pop

4. Identify the bug in this Go Min Stack Pop method:

func (ms *MinStack) Pop() {
  ms.values = ms.values[:len(ms.values)-1]
  ms.mins = ms.mins[:len(ms.mins)]
}

medium

A. Pop method should return the popped value

B. The values slice is not shortened on pop

C. Pop method should clear both slices completely

D. The mins slice is not shortened on pop

Solution

1. Step 1: Analyze slice operations in Pop

   Values slice is shortened by one element correctly.

2. Step 2: Check mins slice operation

   Mins slice is sliced to full length, so it is not shortened, causing mismatch.

3. Final Answer:

   The mins slice is not shortened on pop -> Option D

4. Quick Check:

   mins slice must shrink with values [OK]

Hint: Pop must shrink both values and mins slices equally [OK]

Common Mistakes:

• Forgetting to shrink mins slice
• Assuming Pop returns value
• Clearing slices fully on pop

5. You want to design a Min Stack that supports Push, Pop, Top, and GetMin all in O(1) time. Which approach below correctly handles the minimum tracking when pushing a new element x?

hard

A. Push x to values; push x only if x < current min to mins

B. Push x to values; push min(x, current min) to mins

C. Push x to values; push current min only if x > current min to mins

D. Push x to values; do not update mins slice

Solution

1. Step 1: Understand min tracking logic

   Each push must record the minimum so far to allow O(1) GetMin.

2. Step 2: Compare new element with current min

   Push the smaller of x and current min onto mins slice to keep track.

3. Final Answer:

   Push x to values; push min(x, current min) to mins -> Option B

4. Quick Check:

   Always push min(x, current min) to mins [OK]

Hint: Always push min of new and current min to mins stack [OK]

Common Mistakes:

• Pushing x only if smaller causes missing mins
• Not updating mins on every push
• Ignoring mins slice updates