Trapping Rain Water Using Stack in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind using a stack to solve the Trapping Rain Water problem?

Use a stack to keep track of bars that are bounded by higher bars on both sides. When a higher bar is found, calculate trapped water using the distance and height difference between bars.

beginner

In the stack approach, what does each element in the stack represent?

Each element represents the index of a bar in the height array, stored to help calculate trapped water when a higher bar is encountered.

intermediate

Why do we pop from the stack when the current bar is higher than the top of the stack in the Trapping Rain Water problem?

Because a higher bar on the right means we can calculate trapped water with the popped bar as the bottom, bounded by the current bar and the new top of the stack.

intermediate

What is the time complexity of the Trapping Rain Water solution using a stack?

O(n), because each bar is pushed and popped at most once from the stack.

advanced

Explain how trapped water is calculated between bars using the stack method.

Calculate distance between current bar and new top of stack minus one. Height is minimum of current and top bar heights minus popped bar height. Multiply distance by height to get trapped water volume.

What does the stack store in the Trapping Rain Water problem?

A Indices of bars

B Heights of bars

C Water volumes

D Distances between bars

The stack stores indices of bars to help calculate trapped water when a higher bar is found.

When do we calculate trapped water in the stack approach?

A When current bar is higher than stack top

B When current bar is lower than stack top

C When stack is empty

D At the end of the array only

Trapped water is calculated when the current bar is higher than the top of the stack.

What is the time complexity of the stack-based Trapping Rain Water solution?

A O(n^2)

B O(n)

C O(log n)

D O(n log n)

Each bar is pushed and popped at most once, so the time complexity is O(n).

What does the 'distance' represent when calculating trapped water using the stack?

A Height difference between bars

B Index of the current bar

C Total water trapped so far

D Number of bars between current and stack top minus one

Distance is the horizontal gap between the current bar and the new top of the stack minus one.

Why do we use the minimum height of current and stack top bars in water calculation?

A To calculate the total height of bars

B To find the tallest bar

C To find the lowest boundary for water trapping

D To ignore the popped bar height

The minimum height of the two bars forms the boundary that limits how much water can be trapped.

Describe step-by-step how the stack is used to calculate trapped rain water.

Think about how the stack helps find boundaries for water trapping.

Explain why the stack-based approach is efficient compared to brute force for trapping rain water.

Consider how many times each bar is pushed or popped.