Watering Plants II

Watering Plants II

What is this problem about?

"Watering Plants II" is an evolution of the plant watering simulation. In this version, two friends (let's call them Alice and Bob) start watering a row of plants from opposite ends. Alice starts from the left and Bob starts from the right. They meet in the middle. Like the first problem, they have watering cans with specific capacities. If a friend's can doesn't have enough water for the current plant, they must go back to their respective refill stations at either end. If they reach the same plant simultaneously, the person with the most water takes care of it. The task is to count how many times they need to refill their cans in total.

Why is this asked in interviews?

This problem is a favorite for companies like Google because it introduces the Two Pointers technique within a simulation. It tests whether a candidate can manage two independent processes running toward each other. It requires handling edge cases, specifically the "middle plant" logic when the number of plants is odd. It evaluates your ability to write clean, modular logic for the refill condition that applies to both pointers.

Algorithmic pattern used

The core pattern is the Two Pointers approach combined with Simulation. You maintain one pointer at index 0 and another at index N-1. You increment the left pointer and decrement the right pointer until they meet. This ensures the entire array is processed in a single pass, making it an efficient O(N) solution.

Example explanation

Consider plants with needs [3, 2, 4, 3] and both friends having capacity 5.

• Alice starts at index 0 (needs 3). She has 5, waters it. Remaining: 2. Refills: 0.
• Bob starts at index 3 (needs 3). He has 5, waters it. Remaining: 2. Refills: 0.
• Alice moves to index 1 (needs 2). She has 2, waters it. Remaining: 0. Refills: 0.
• Bob moves to index 2 (needs 4). He has 2, but needs 4. He refills! Remaining: 5 - 4 = 1. Refills: 1. Total refills: 1.

Common mistakes candidates make

• Middle plant logic: Failing to correctly handle the case where both pointers meet at the exact same plant. The rule usually states the one with more water goes first, or if they are equal, the first person (Alice) goes.
• Independent refill counters: Trying to use a single water variable for both friends. Each must have their own state.
• Loop termination: Ending the loop too early and missing the middle plant.

Interview preparation tip

For Two Pointers interview patterns, always consider the odd vs. even length of the input array. Use a while loop with left < right and then handle the left == right case separately after the loop to ensure clarity and correctness for the middle element.