Trapping Rain Water Using Stack in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the stack pointer for indices.

DSA C

int stack[100];
int top = [1];

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

A0

B100

C-1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing top to 0 causes off-by-one errors.

Using 100 as top index causes out-of-bounds.

The stack top is initialized to -1 to indicate an empty stack.

2fill in blank

medium

Complete the code to push an index onto the stack.

DSA C

stack[++top] = [1];

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

Ai

Bstack[top]

Ctop

Dheight[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Pushing height[i] instead of index causes logic errors.

Pushing top or stack[top] causes wrong stack content.

We push the current index i onto the stack, not the height value.

3fill in blank

hard

Fix the error in calculating the distance between bars.

DSA C

distance = i - stack[[1]] - 1;

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

Atop

Btop + 1

Ctop - 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using top - 1 calculates overly wide distance.

Using top + 1 causes out-of-bounds access.

Distance is between current index i and stack[top] (left bounding bar), so top is correct.

4fill in blank

hard

Fill both blanks to calculate bounded height and update trapped water.

DSA C

bounded_height = (height[i] < height[stack[top]]) ? height[i] - height[stack[[1]]] : height[stack[top]] - height[stack[[2]]];
water += distance * bounded_height;

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

Atop - 1

Btop

Ctop + 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using top subtracts left height (zero water).

Using top - 1 subtracts wrong bar height.

bounded_height is min(height[i], height[stack[top]]) minus height[stack[top + 1]] (popped bar index).

5fill in blank

hard

Fill all three blanks to complete the main loop for trapping rain water.

DSA C

for (int i = 0; i < n; i++) {
    while (top != -1 && height[i] > height[stack[top]]) {
        int top_index = stack[top--];
        if (top == -1) continue;
        int distance = i - stack[[1]] - 1;
        int bounded_height = (height[i] < height[stack[top]]) ? height[i] - height[stack[[2]]] : height[stack[top]] - height[stack[[3]]];
        water += distance * bounded_height;
    }
    stack[++top] = i;
}

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

Atop

Btop - 1

Ctop + 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using top - 1 for distance causes wrong width.

Swapping indices in bounded_height causes wrong height calculation.

Distance uses top (left index). bounded_height uses top + 1 (popped index) correctly.