Trapping Rain Water Using Stack in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the stack used for indices.

DSA C++

std::stack<int> [1];

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

Abars

Bwater

Cindices

Dheights

Attempts:

3 left

💡 Hint

Common Mistakes

Using stack of heights instead of indices

Naming stack as 'water' which is confusing

The stack stores indices of bars to calculate trapped water.

2fill in blank

medium

Complete the code to get the current height from the input vector.

DSA C++

int currentHeight = height[[1]];

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

Ai

Bj

Ctop

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' which is a stack variable, not loop index

Using 'j' which is undefined here

We use 'i' as the loop variable to access current height.

3fill in blank

hard

Fix the error in the while loop condition to check stack and height.

DSA C++

while (!indices.empty() && height[[1]] > height[indices.top()]) {

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

Ai

Btop

Cindices.top()

Dheight[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' as index which is a method call

Using 'indices.top()' inside height[] which is correct but not the blank here

We compare current height at i with height at top index of stack.

4fill in blank

hard

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

DSA C++

int distance = i - [1] - 1;
int bounded_height = std::min(height[i], height[[2]]) - height[top];

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

Aindices.top()

Btop

Ci

Dindices.pop()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' instead of 'indices.top()' for distance

Using 'top' instead of 'indices.top()' for bounded height calculation

Distance is between current index i and stack top index; bounded height uses min of current and new top heights.

5fill in blank

hard

Fill all three blanks to update total trapped water and push current index.

DSA C++

water += distance * bounded_height;
indices.[1]();
indices.[2](i);
return [3];

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

Apop

Bpush

Cwater

DtotalWater

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' instead of 'push' or vice versa

Returning wrong variable name like 'totalWater' instead of 'water'

We pop the top index, push current index i, and return total water stored in 'water'.

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

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 order of the bars

Solution

1. Step 1: Understand the role of the stack

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

2. Step 2: Explain why other options are incorrect

   The stack does not store water amount or sort bars; it helps track bars for water calculation.

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 = A [OK]

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

Common Mistakes:

• Thinking stack stores water amount
• Assuming stack sorts bars
• Confusing stack with array for heights

2. Which of the following C++ code snippets correctly declares a stack to be used for trapping rain water indices?

easy

A. std::stack<int> st;

B. stack<int> st = new stack<int>();

C. std::stack st<int>;

D. stack<int> st();

Solution

1. Step 1: Check correct C++ stack declaration syntax

   Correct syntax is std::stack<int> st; to declare a stack of integers.

2. Step 2: Identify errors in other options

   stack<int> st = new stack<int>(); uses incorrect syntax with 'new'; C has wrong template placement; D declares a function, not a stack.

3. Final Answer:

   std::stack<int> st; -> Option A

4. Quick Check:

   Correct stack declaration = B [OK]

Hint: Use std::stack<int> st; to declare stack [OK]

Common Mistakes:

• Using 'new' keyword incorrectly
• Wrong template syntax
• Declaring function instead of stack

3. What is the output of the following C++ code snippet that uses stack to calculate trapped rain water?

int height[] = {0,1,0,2,1,0,1,3,2,1,2,1};
int n = 12;
std::stack<int> st;
int water = 0;
for (int i = 0; i < n; i++) {
  while (!st.empty() && height[i] > height[st.top()]) {
    int top = st.top();
    st.pop();
    if (st.empty()) break;
    int distance = i - st.top() - 1;
    int bounded_height = std::min(height[i], height[st.top()]) - height[top];
    water += distance * bounded_height;
  }
  st.push(i);
}
std::cout << water << std::endl;

medium

A. 8

B. 5

C. 6

D. 7

Solution

1. Step 1: Trace the stack operations and water calculation

   The code calculates trapped water for the given heights array using stack logic. The total trapped water is 6 units.

2. Step 2: Verify calculations with known example

   This is a classic example where trapped water is 6 units, matching the output of the code.

3. Final Answer:

   6 -> Option C

4. Quick Check:

   Trapped water = 6 [OK]

Hint: Classic example traps 6 units water [OK]

Common Mistakes:

• Miscounting distance between bars
• Wrong bounded height calculation
• Ignoring empty stack break condition

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

std::stack<int> st;
int water = 0;
for (int i = 0; i < height.size(); i++) {
  while (!st.empty() && height[i] >= height[st.top()]) {
    int top = st.top();
    st.pop();
    if (st.empty()) break;
    int distance = i - st.top() - 1;
    int bounded_height = std::min(height[i], height[st.top()]) - height[top];
    water += distance * bounded_height;
  }
  st.push(i);
}
std::cout << water << std::endl;

medium

A. No break condition inside while loop

B. Missing initialization of water variable

C. Stack should store heights, not indices

D. Using >= instead of > in while condition causes incorrect water calculation

Solution

1. Step 1: Analyze the while loop condition

   The condition uses >= which causes popping bars of equal height, leading to incorrect trapped water calculation.

2. Step 2: Explain correct condition and impact

   Using > ensures only bars lower than current are popped, preserving correct boundaries for water trapping.

3. Final Answer:

   Using >= instead of > in while condition causes incorrect water calculation -> Option D

4. Quick Check:

   Use > not >= in while condition [OK]

Hint: Use > not >= in while condition [OK]

Common Mistakes:

• Using >= instead of > in while condition
• Storing heights instead of indices in stack
• Forgetting break when stack is empty

5. Given the heights array {4, 2, 0, 3, 2, 5}, what is the total trapped rain water calculated using the stack approach?

hard

A. 8

B. 7

C. 9

D. 6

Solution

1. Step 1: Calculate trapped water between bars using stack logic

   Between bars, trapped water units are: 2 units between 4 and 3, 1 unit between 3 and 5, and 5 units between 2 and 5, totaling 8 units.

2. Step 2: Sum trapped water units

   Sum of trapped water is 8 units for the given array.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   Trapped water total = 8 [OK]

Hint: Sum water trapped between boundaries carefully [OK]

Common Mistakes:

• Ignoring zero height bars
• Miscounting trapped water between bars
• Not considering both left and right boundaries