Trapping Rain Water Using Stack in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a stack for storing indices.

DSA Java

Stack<Integer> stack = new Stack<>();
int i = 0;
while (i < height.length) {
    if (stack.isEmpty() || height[i] [1] height[stack.peek()]) {
        stack.push(i++);
    } else {
        int top = stack.pop();
        // Calculate trapped water here
    }
}

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

A>

B<=

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' causes incorrect stack behavior.

Using '==' does not handle all cases properly.

We push the current index if the current height is less than the height at the top index of the stack.

2fill in blank

medium

Complete the code to calculate the distance between current index and stack top after popping.

DSA Java

int distance = i - stack.peek() - [1];

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

A0

B-1

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or 2 causes wrong distance calculation.

Using -1 leads to negative distance.

Distance is the difference between current index and stack top minus 1.

3fill in blank

hard

Fix the error in calculating bounded height for trapped water.

DSA Java

int bounded_height = Math.min(height[i], height[stack.peek()]) - height[[1]];

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

Ai

Btop

Cstack.peek()

Dheight[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'i' or 'stack.peek()' instead of 'top' causes wrong height calculation.

Using 'height[i]' is incorrect as it is a value, not an index.

We subtract the height of the popped top bar to get the bounded height.

4fill in blank

hard

Fill both blanks to update total trapped water correctly.

DSA Java

water += distance * [1];
i++;

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

Abounded_height

Bheight[top]

Cheight[i]

Dstack.peek()

Attempts:

3 left

💡 Hint

Common Mistakes

Using height[top] or height[i] directly causes wrong water calculation.

Using stack.peek() is incorrect as it is an index, not height.

Total trapped water increases by distance times bounded height.

5fill in blank

hard

Fill all three blanks to complete the trapping rain water function using stack.

DSA Java

public int trap(int[] height) {
    int water = 0;
    Stack<Integer> stack = new Stack<>();
    int i = 0;
    while (i < height.length) {
        if (stack.isEmpty() || height[i] [1] height[stack.peek()]) {
            stack.push(i++);
        } else {
            int top = stack.pop();
            if (stack.isEmpty()) continue;
            int distance = i - stack.peek() - [2];
            int bounded_height = Math.min(height[i], height[stack.peek()]) - height[[3]];
            water += distance * bounded_height;
        }
    }
    return water;
}

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

A<

B1

Ctop

D>

Attempts:

3 left

💡 Hint

Common Mistakes

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

Incorrect distance calculation by subtracting wrong value.

Using wrong variable for height subtraction.

The function uses a stack to find trapped water by comparing heights and calculating bounded areas.

Practice

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

easy

A. To keep track of the indices of bars that are not yet processed for water trapping

B. To store the heights of the bars in sorted order

C. To reverse the array of bar heights

D. To count the total number of 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 how the stack helps in trapping water

   When a bar taller than the top of the stack is found, it means water can be trapped between bars. The stack helps find the left boundary and calculate trapped water.

3. Final Answer:

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

4. Quick Check:

   Stack stores indices for trapping water = C [OK]

Hint: Stack holds indices to find boundaries for trapped water [OK]

Common Mistakes:

• Thinking stack stores heights instead of indices
• Assuming stack sorts bars by height
• Confusing stack use with reversing the array

2. Which of the following Java code snippets correctly initializes an empty stack to be used for trapping rain water indices?

easy

A. Stack stack = new Stack<int>();

B. Stack<Integer> stack = new Stack<>();

C. Stack<int> stack = new Stack<>();

D. Stack<Integer> stack = new Stack<int>();

Solution

1. Step 1: Check Java generic syntax for Stack

   Java uses generics with angle brackets and wrapper classes like Integer, not primitive int.

2. Step 2: Identify correct initialization

   Correct syntax is Stack<Integer> stack = new Stack<>(); which uses Integer wrapper and diamond operator.

3. Final Answer:

   Stack<Integer> stack = new Stack<>(); -> Option B

4. Quick Check:

   Use wrapper Integer and diamond operator = A [OK]

Hint: Use Stack<Integer> with diamond operator for empty stack [OK]

Common Mistakes:

• Using primitive int inside generics
• Omitting generic type causing raw type warning
• Incorrect constructor syntax with generics

3. What is the output of the following Java code snippet that calculates trapped rain water using stack?

int[] height = {0,1,0,2,1,0,1,3,2,1,2,1};
System.out.println(trap(height));

public static int trap(int[] height) {
    Stack<Integer> stack = new Stack<>();
    int water = 0, current = 0;
    while (current < height.length) {
        while (!stack.isEmpty() && height[current] > height[stack.peek()]) {
            int top = stack.pop();
            if (stack.isEmpty()) break;
            int distance = current - stack.peek() - 1;
            int boundedHeight = Math.min(height[current], height[stack.peek()]) - height[top];
            water += distance * boundedHeight;
        }
        stack.push(current++);
    }
    return water;
}

medium

A. 5

B. 7

C. 6

D. 8

Solution

1. Step 1: Trace the stack and water calculation

   The code uses a stack to find boundaries and calculates trapped water step-by-step for the given heights.

2. Step 2: Calculate trapped water for given input

   For the input array, the total trapped water calculated is 6 units after processing all bars.

3. Final Answer:

   6 -> Option C

4. Quick Check:

   Trapped water for given heights = 6 [OK]

Hint: Trace stack pops and water increments carefully [OK]

Common Mistakes:

• Miscounting distance between bars
• Incorrect bounded height calculation
• Forgetting to break when stack is empty

4. Identify the error in the following Java code snippet for trapping rain water using stack and select the fix.

public static int trap(int[] height) {
    Stack<Integer> stack = new Stack<>();
    int water = 0, current = 0;
    while (current <= height.length) {
        while (!stack.isEmpty() && height[current] > height[stack.peek()]) {
            int top = stack.pop();
            if (stack.isEmpty()) break;
            int distance = current - stack.peek() - 1;
            int boundedHeight = Math.min(height[current], height[stack.peek()]) - height[top];
            water += distance * boundedHeight;
        }
        stack.push(current++);
    }
    return water;
}

medium

A. Change 'stack.pop()' to 'stack.peek()' to avoid removing elements

B. Replace Stack<Integer> with ArrayList<Integer> for better performance

C. Remove the inner while loop to simplify the code

D. Change 'current <= height.length' to 'current < height.length' to avoid ArrayIndexOutOfBoundsException

Solution

1. Step 1: Identify the loop boundary error

   The condition 'current <= height.length' causes an attempt to access height[height.length], which is out of bounds.

2. Step 2: Fix the loop condition

   Changing it to 'current < height.length' prevents the out-of-bounds error and allows correct iteration.

3. Final Answer:

   Change 'current <= height.length' to 'current < height.length' to avoid ArrayIndexOutOfBoundsException -> Option D

4. Quick Check:

   Loop boundary must be less than array length = A [OK]

Hint: Use '<' for array index loops to avoid out-of-bounds [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Replacing stack with wrong data structure
• Removing necessary inner loop

5. Given the array height = {4, 2, 0, 3, 2, 5}, what is the total amount of trapped rain water calculated by the stack-based algorithm?

hard

A. 9

B. 7

C. 10

D. 8

Solution

1. Step 1: Analyze trapped water between bars

   Between bars of height 4 and 5, water is trapped over lower bars: 2, 0, 3, 2.

2. Step 2: Calculate trapped water volume

   Water level is min(4,5)=4. Water trapped = (4-2) + (4-0) + (4-3) + (4-2) = 2 + 4 + 1 + 2 = 9 units.

3. Final Answer:

   9 -> Option A

4. Quick Check:

   Sum trapped water between boundaries = 9 [OK]

Hint: Calculate water trapped between boundaries using min height [OK]

Common Mistakes:

• Using left boundary height only
• Ignoring right boundary taller bar
• Adding heights instead of differences