Trapping Rain Water Using Stack in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the stack as an empty array.

DSA Javascript

const stack = [1];

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

A0

B{}

Cnull

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which creates an object, not an array.

Using null or 0 which are not arrays.

The stack is initialized as an empty array to store indices.

2fill in blank

medium

Complete the code to push the current index i onto the stack.

DSA Javascript

stack.[1](i);

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

Apush

Bshift

Cpop

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes elements instead of adding.

Using shift or unshift which operate on the start of the array.

The push method adds an element to the end of the array (stack).

3fill in blank

hard

Fix the error in the while loop condition to check if the stack is not empty.

DSA Javascript

while (stack.[1] > 0 && height[i] > height[stack[stack.length - 1]]) {

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

Alength()

Blength

Ccount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() which causes a runtime error.

Using size or count which are not valid for arrays.

Arrays in JavaScript use the length property (without parentheses) to get size.

4fill in blank

hard

Fill both blanks to calculate the distance and bounded height for trapped water.

DSA Javascript

const distance = i - stack[stack.length - 1] - [1];
const bounded_height = Math.min(height[i], height[stack[stack.length - 1]]) - height[[2]];

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

A1

B0

Ctop

Dstack.pop()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 for distance calculation.

Using stack[stack.length - 1] instead of the popped index for height.

Distance is the gap between current and previous index minus 1. Bounded height uses the popped index from stack.

5fill in blank

hard

Fill all three blanks to update the total trapped water correctly.

DSA Javascript

water += distance * [1];
stack.[2](i);
let top = stack.[3]();

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

Abounded_height

Bpush

Cpop

Dheight[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using height[i] instead of bounded_height for water calculation.

Using pop instead of push to add index.

Using push instead of pop to remove index.

Water is increased by distance times bounded height. Push adds current index. Pop removes top index.

Practice

1. What is the main purpose of using a stack in the Trapping Rain Water Using Stack algorithm?

easy

A. To store the total amount of trapped water directly

B. To keep track of bars that are not yet processed for water trapping

C. To sort the heights of bars in ascending order

D. To reverse the array of bar heights

Solution

1. Step 1: Understand the role of stack in the algorithm

   The stack stores indices of bars that are waiting to find a right boundary to trap water.

2. Step 2: Clarify what the stack does not do

   The stack does not store water amounts or sort bars; it helps find boundaries for trapped water.

3. Final Answer:

   To keep track of bars that are not yet processed for water trapping -> Option B

4. Quick Check:

   Stack tracks bars for trapping water [OK]

Hint: Stack holds indices of bars waiting for right boundary [OK]

Common Mistakes:

• Thinking stack stores water volume directly
• Assuming stack sorts the bars
• Believing stack reverses the array

2. Which of the following is the correct way to initialize an empty stack in JavaScript for the trapping rain water problem?

easy

A. const stack = [];

B. const stack = {};

C. const stack = null;

D. const stack = new Stack();

Solution

1. Step 1: Identify the correct data structure for stack in JavaScript

   JavaScript uses arrays as stacks by pushing and popping elements.

2. Step 2: Check each option

   const stack = []; initializes an empty array, which works as a stack. const stack = {}; is an object, not a stack. const stack = null; is null, no stack. const stack = new Stack(); assumes a Stack class which is not built-in.

3. Final Answer:

   const stack = []; -> Option A

4. Quick Check:

   Empty array for stack [OK]

Hint: Use empty array [] as stack in JavaScript [OK]

Common Mistakes:

• Using object {} instead of array
• Assuming Stack class exists by default
• Initializing stack as null

3. What will be the output of the following code snippet that uses stack to calculate trapped rain water?

const height = [0,1,0,2,1,0,1,3,2,1,2,1];
function trap(height) {
  let stack = [];
  let water = 0;
  for (let i = 0; i < height.length; i++) {
    while (stack.length && height[i] > height[stack[stack.length - 1]]) {
      let top = stack.pop();
      if (!stack.length) break;
      let distance = i - stack[stack.length - 1] - 1;
      let boundedHeight = Math.min(height[i], height[stack[stack.length - 1]]) - height[top];
      water += distance * boundedHeight;
    }
    stack.push(i);
  }
  return water;
}
console.log(trap(height));

medium

A. 6

B. 5

C. 7

D. 8

Solution

1. Step 1: Understand the input and algorithm flow

   The input array represents bar heights. The stack-based algorithm calculates trapped water by finding boundaries and distances.

2. Step 2: Trace the code logic mentally

   The code correctly calculates trapped water for the given heights, which is a classic example resulting in 6 units of water trapped.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Classic trapping rain water example = 6 [OK]

Hint: Classic example height array traps 6 units water [OK]

Common Mistakes:

• Miscounting trapped water units
• Ignoring stack empty check
• Wrong distance or bounded height calculation

4. Identify the error in the following code snippet for trapping rain water using stack:

function trap(height) {
  let stack = [];
  let water = 0;
  for (let i = 0; i < height.length; i++) {
    while (stack.length && height[i] >= height[stack[stack.length - 1]]) {
      let top = stack.pop();
      if (!stack.length) break;
      let distance = i - stack[stack.length - 1] - 1;
      let boundedHeight = Math.min(height[i], height[stack[stack.length - 1]]) - height[top];
      water += distance * boundedHeight;
    }
    stack.push(i);
  }
  return water;
}

medium

A. The distance calculation is incorrect

B. The stack should be initialized as an object, not array

C. The comparison should be >, not >= in the while condition

D. The water variable should be reset inside the loop

Solution

1. Step 1: Analyze the while loop condition

   The condition uses >= which causes incorrect popping when heights are equal, leading to wrong trapped water calculation.

2. Step 2: Correct the condition

   Changing >= to > ensures only strictly higher bars trigger popping, preserving correct boundaries.

3. Final Answer:

   The comparison should be >, not >= in the while condition -> Option C

4. Quick Check:

   Use > in while condition to avoid wrong pops [OK]

Hint: Use >, not >=, to avoid popping equal height bars [OK]

Common Mistakes:

• Using >= causes incorrect water calculation
• Initializing stack as object instead of array
• Resetting water inside loop wrongly

5. Given the heights array [4, 2, 0, 3, 2, 5], what is the total trapped rain water calculated using the stack method?

hard

A. 6

B. 7

C. 8

D. 9

Solution

1. Step 1: Visualize the bars and trapped water

   Bars heights: 4,2,0,3,2,5. Water trapped between bars is calculated by stack method considering boundaries and distances.

2. Step 2: Calculate trapped water step-by-step

   Water at index 1: 2, index 2: 4, index 3: 1, index 4: 2, total 9 units.

3. Final Answer:

   9 -> Option D

4. Quick Check:

   Sum trapped water units = 9 [OK]

Hint: Sum water between boundaries carefully for correct total [OK]

Common Mistakes:

• Miscounting trapped water units
• Ignoring zero height bars
• Wrong boundary identification