Container With Most Water in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the left pointer at the start of the array.

DSA Go

left := [1]

Drag options to blanks, or click blank then click option'

A-1

B1

Clen(height) - 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting left pointer at the end of the array.

Using negative index for left pointer.

The left pointer should start at index 0, the beginning of the array.

2fill in blank

medium

Complete the code to initialize the right pointer at the end of the array.

DSA Go

right := [1]

Drag options to blanks, or click blank then click option'

A0

Blen(height) - 1

Clen(height)

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using length as index (out of range).

Starting right pointer at zero.

The right pointer should start at the last index, which is length minus one.

3fill in blank

hard

Fix the error in the condition to calculate the container area correctly.

DSA Go

area := min(height[left], height[right]) * [1]

Drag options to blanks, or click blank then click option'

Aright - left

Bleft + right

Cleft - right

Dleft * right

Attempts:

3 left

💡 Hint

Common Mistakes

Adding left and right indices instead of subtracting.

Using multiplication of indices for width.

The width between pointers is the difference of right and left indices.

4fill in blank

hard

Fill both blanks to move the pointers correctly after calculating area.

DSA Go

if height[left] < height[right] {
    left [1]
} else {
    right [2]
}

Drag options to blanks, or click blank then click option'

A++

B--

C+=

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using decrement on left pointer.

Using increment on right pointer.

If left height is smaller, move left pointer right by incrementing. Otherwise, move right pointer left by decrementing.

5fill in blank

hard

Fill all three blanks to update max area correctly inside the loop.

DSA Go

if area [1] maxArea {
    maxArea = [2]
}

// Move pointers
if height[left] < height[right] {
    left [3]
} else {
    right--
}

Drag options to blanks, or click blank then click option'

A>

Barea

C++

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in comparison.

Not updating maxArea correctly.

Moving wrong pointer incorrectly.

Update maxArea if current area is greater. Then move left pointer right if left height is smaller.

Practice

1. What does the problem Container With Most Water ask you to find given an array of heights?

easy

A. The sum of all heights

B. The minimum height in the array

C. The maximum area of water that can be contained between two lines

D. The number of lines taller than a given height

Solution

1. Step 1: Understand the problem statement

   The problem asks to find two lines which together with the x-axis form a container that holds the most water.

2. Step 2: Identify what to calculate

   We calculate the area between two lines as the width between them times the smaller height.

3. Final Answer:

   The maximum area of water that can be contained between two lines -> Option C

4. Quick Check:

   Container with most water = maximum area [OK]

Hint: Focus on max area between two heights, not sum or count [OK]

Common Mistakes:

• Confusing max area with sum of heights
• Ignoring the smaller height when calculating area
• Thinking about counting lines instead of area

2. Which of the following Go code snippets correctly declares a slice of heights for the Container With Most Water problem?

easy

A. heights := []int{1, 8, 6, 2, 5, 4, 8, 3, 7}

B. heights := [1, 8, 6, 2, 5, 4, 8, 3, 7]

C. heights := (1, 8, 6, 2, 5, 4, 8, 3, 7)

D. heights := {1, 8, 6, 2, 5, 4, 8, 3, 7}

Solution

1. Step 1: Recall Go slice declaration syntax

   In Go, slices are declared using []type{values}, for example []int{1,2,3}.

2. Step 2: Check each option

   heights := []int{1, 8, 6, 2, 5, 4, 8, 3, 7} uses []int{...} which is correct. Options A, B, C use invalid syntax for slices.

3. Final Answer:

   heights := []int{1, 8, 6, 2, 5, 4, 8, 3, 7} -> Option A

4. Quick Check:

   Slice declaration uses []type{values} [OK]

Hint: Use []int{...} to declare slices in Go [OK]

Common Mistakes:

• Using parentheses or braces instead of [] for slices
• Confusing arrays and slices syntax
• Missing type declaration in slice

3. What is the output of the following Go code snippet that implements the Container With Most Water logic?

package main
import "fmt"
func maxArea(height []int) int {
    left, right := 0, len(height)-1
    maxArea := 0
    for left < right {
        width := right - left
        var h int
        if height[left] < height[right] {
            h = height[left]
            left++
        } else {
            h = height[right]
            right--
        }
        area := width * h
        if area > maxArea {
            maxArea = area
        }
    }
    return maxArea
}
func main() {
    heights := []int{1,8,6,2,5,4,8,3,7}
    fmt.Println(maxArea(heights))
}

medium

A. 49

B. 56

C. 48

D. 64

Solution

1. Step 1: Understand the two-pointer approach

   Start with left=0 and right=8, calculate area and move the pointer with smaller height inward.

2. Step 2: Trace the iterations

   Initial area: min(1,7)*8=8 Move left to 1 Next area: min(8,7)*7=49 Move right to 7 Next area: min(8,3)*6=18 Move right to 6 Next area: min(8,8)*5=40 Move right to 5 Next area: min(8,4)*4=16 Move right to 4 Next area: min(8,5)*3=15 Move right to 3 Next area: min(8,2)*2=4 Move right to 2 Next area: min(8,6)*1=1 Move left to 2 Max area found is 49.

3. Final Answer:

   49 -> Option A

4. Quick Check:

   Max area calculated by two-pointer = 49 [OK]

Hint: Use two pointers moving inward, track max area [OK]

Common Mistakes:

• Calculating area with max height instead of min
• Not moving the pointer with smaller height
• Confusing width calculation

4. Identify the bug in this Go function for Container With Most Water and select the fix:

func maxArea(height []int) int {
    left, right := 0, len(height)-1
    maxArea := 0
    for left <= right {
        width := right - left
        var h int
        if height[left] < height[right] {
            h = height[left]
            left++
        } else {
            h = height[right]
            right--
        }
        area := width * h
        if area > maxArea {
            maxArea = area
        }
    }
    return maxArea
}

medium

A. Use maxArea = area without checking if area > maxArea

B. Initialize maxArea to -1 instead of 0

C. Increment both left and right pointers inside the loop

D. Change loop condition to 'left < right' instead of 'left <= right'

Solution

1. Step 1: Analyze the loop condition

   The loop runs while left <= right, which allows left and right to be equal, causing width to be zero or negative.

2. Step 2: Fix the loop condition

   Changing to left < right ensures width is always positive and valid for area calculation.

3. Final Answer:

   Change loop condition to 'left < right' instead of 'left <= right' -> Option D

4. Quick Check:

   Loop must run while left < right [OK]

Hint: Use strict less than for pointers to avoid zero width [OK]

Common Mistakes:

• Using <= instead of < causing invalid area
• Updating both pointers at once
• Not checking if area is greater before updating maxArea

5. Given the heights slice []int{4, 3, 2, 1, 4}, what is the maximum water container area? Use the two-pointer approach and explain why the maximum area is found at those positions.

hard

A. 12

B. 16

C. 8

D. 6

Solution

1. Step 1: Initialize pointers and calculate areas

   Start with left=0 (height=4), right=4 (height=4), width=4, area=4*4=16.

2. Step 2: Move pointers and check other areas

   Since heights[left] == heights[right], move right pointer left: Now left=0, right=3 (height=1), width=3, area=min(4,1)*3=3 Move right to 2 (height=2), area= min(4,2)*2=4 Move right to 1 (height=3), area= min(4,3)*1=3 Max area remains 16.

3. Final Answer:

   16 -> Option B

4. Quick Check:

   Max area between positions 0 and 4 = 16 [OK]

Hint: Max area often at widest distance with min height [OK]

Common Mistakes:

• Not considering both ends first
• Moving wrong pointer when heights are equal
• Calculating area incorrectly with wrong width