Trapping Rain Water Problem in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of the Trapping Rain Water Problem?

To find how much water can be trapped between bars of different heights after raining.

beginner

What does the 'left max' array represent in the Trapping Rain Water Problem?

It stores the highest bar height from the left up to each position, helping to calculate trapped water.

intermediate

Why do we use two pointers in the optimized solution of the Trapping Rain Water Problem?

To scan from both ends towards the center, reducing space usage and improving efficiency.

intermediate

Explain how trapped water at a position is calculated.

Trapped water = minimum of max heights on left and right minus the height at that position, if positive.

intermediate

What is the time and space complexity of the two-pointer approach for this problem?

Time complexity is O(n) and space complexity is O(1), where n is the number of bars.

What does the Trapping Rain Water Problem calculate?

A Total length of bars

B Maximum height of bars

C Number of bars

D Amount of water trapped between bars

The problem calculates how much water can be trapped between bars after raining.

Which data structure is commonly used to store max heights from left or right?

A Stack

B Array

C Queue

D Linked List

Arrays are used to store max heights from left and right for each position.

In the two-pointer approach, what determines which pointer to move?

A Pointer with smaller max height moves

B Pointer with larger max height moves

C Always move left pointer

D Always move right pointer

The pointer with smaller max height moves inward to find trapped water.

What is the space complexity of the naive approach using left and right max arrays?

A O(n)

B O(n^2)

C O(1)

D O(log n)

The naive approach uses two extra arrays of size n, so space complexity is O(n).

Which of these is NOT a step in calculating trapped water at a position?

A Find minimum of left max and right max

B Subtract height at position

C Add height at position

D Use zero if result is negative

We subtract the height at the position, not add it, to find trapped water.

Describe the two-pointer approach to solve the Trapping Rain Water Problem.

Think about scanning from both ends and comparing heights.

Explain how to calculate trapped water at a single position using max heights.

Water level depends on the shorter boundary around the bar.