Watering Plants

Watering Plants

What is this problem about?

The "Watering Plants" problem is a classic array-based simulation challenge. You are given an array of integers representing the water requirements of different plants arranged in a row. You have a watering can with a fixed capacity. Starting from the river (at position -1), you walk to each plant in order. If you have enough water to satisfy the current plant's needs, you water it and move to the next. However, if your watering can is insufficient, you must walk all the way back to the river to refill it to full capacity before returning to the current plant. The goal is to calculate the total number of steps taken to water all the plants.

Why is this asked in interviews?

This problem is popular in technical interviews at companies like Google because it tests a candidate's ability to translate a real-world scenario into a clean simulation. It isn't about complex data structures but rather about careful logic and understanding "state." Interviewers look for how well you handle the transition between states (having enough water vs. needing a refill) and whether you can correctly manage the step counter without off-by-one errors. It's a great test of attention to detail.

Algorithmic pattern used

The primary pattern used here is Simulation with a Single Pass through the array. You maintain two variables: one for the current amount of water in your can and another for the total steps taken. Since you visit each plant in a linear fashion, the time complexity is O(N), where N is the number of plants.

Example explanation

Imagine three plants with water needs [2, 4, 3] and a watering can capacity of 5.

1. Start at the river (position -1).
2. Walk to Plant 0 (1 step). Need 2 units, have 5. Remaining: 3. Total steps: 1.
3. Walk to Plant 1 (1 step). Need 4 units, have 3. Not enough!
4. Walk back to the river from Plant 0 position (1 step). Refill. Walk back to Plant 1 (2 steps). Remaining: 5 - 4 = 1. Total steps: 1 + 1 + 2 = 4.
5. Walk to Plant 2 (1 step). Need 3 units, have 1. Not enough!
6. Walk back to the river from Plant 1 position (2 steps). Refill. Walk back to Plant 2 (3 steps). Remaining: 5 - 3 = 2. Total steps: 4 + 2 + 3 = 9. Final result: 9 steps.

Common mistakes candidates make

• Off-by-one errors: Forgetting that the river is at index -1 and calculating distances incorrectly.
• Incorrect refill distance: Miscalculating how many steps it takes to go back to the river and return to the current plant. Remember, you move to the plant position after refilling.
• Updating water late: Forgetting to subtract the water used after a refill.

Interview preparation tip

When tackling simulation problems like the Watering Plants coding problem, always dry-run your logic with a small, concrete example before writing code. Tracking the state of your variables (water level and steps) on paper helps you spot the distance calculation patterns clearly.