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Watering Plants II

Alice and Bob want to water n plants in their garden. The plants are arranged in a row and are labeled from 0 to n - 1 from left to right where the ith plant is located at x = i.

Each plant needs a specific amount of water. Alice and Bob have a watering can each, initially full. They water the plants in the following way:

  • Alice waters the plants in order from left to right, starting from the 0th plant. Bob waters the plants in order from right to left, starting from the (n - 1)th plant. They begin watering the plants simultaneously.
  • It takes the same amount of time to water each plant regardless of how much water it needs.
  • Alice/Bob must water the plant if they have enough in their can to fully water it. Otherwise, they first refill their can (instantaneously) then water the plant.
  • In case both Alice and Bob reach the same plant, the one with more water currently in his/her watering can should water this plant. If they have the same amount of water, then Alice should water this plant.

Given a 0-indexed integer array plants of n integers, where plants[i] is the amount of water the ith plant needs, and two integers capacityA and capacityB representing the capacities of Alice's and Bob's watering cans respectively, return the number of times they have to refill to water all the plants.

 

Example 1:

Input: plants = [2,2,3,3], capacityA = 5, capacityB = 5
Output: 1
Explanation:
- Initially, Alice and Bob have 5 units of water each in their watering cans.
- Alice waters plant 0, Bob waters plant 3.
- Alice and Bob now have 3 units and 2 units of water respectively.
- Alice has enough water for plant 1, so she waters it. Bob does not have enough water for plant 2, so he refills his can then waters it.
So, the total number of times they have to refill to water all the plants is 0 + 0 + 1 + 0 = 1.

Example 2:

Input: plants = [2,2,3,3], capacityA = 3, capacityB = 4
Output: 2
Explanation:
- Initially, Alice and Bob have 3 units and 4 units of water in their watering cans respectively.
- Alice waters plant 0, Bob waters plant 3.
- Alice and Bob now have 1 unit of water each, and need to water plants 1 and 2 respectively.
- Since neither of them have enough water for their current plants, they refill their cans and then water the plants.
So, the total number of times they have to refill to water all the plants is 0 + 1 + 1 + 0 = 2.

Example 3:

Input: plants = [5], capacityA = 10, capacityB = 8
Output: 0
Explanation:
- There is only one plant.
- Alice's watering can has 10 units of water, whereas Bob's can has 8 units. Since Alice has more water in her can, she waters this plant.
So, the total number of times they have to refill is 0.

 

Constraints:

  • n == plants.length
  • 1 <= n <= 105
  • 1 <= plants[i] <= 106
  • max(plants[i]) <= capacityA, capacityB <= 109

Solution Explanation: Watering Plants II

This problem involves simulating the process of Alice and Bob watering plants from opposite ends. The key is efficiently tracking their water levels and refills. A two-pointer approach significantly optimizes the solution.

Approach:

  1. Initialization:

    • a: Alice's water level (starts at capacityA).
    • b: Bob's water level (starts at capacityB).
    • ans: Number of refills (initialized to 0).
    • i: Pointer for Alice (starts at the beginning of plants).
    • j: Pointer for Bob (starts at the end of plants).

  2. Iteration: The while i < j loop simulates the simultaneous watering.

    • Alice's Turn:
      • If a < plants[i], Alice needs a refill (ans++), and a is reset to capacityA.
      • Alice waters the plant (a -= plants[i]).
      • i increments.
    • Bob's Turn:
      • If b < plants[j], Bob needs a refill (ans++), and b is reset to capacityB.
      • Bob waters the plant (b -= plants[j]).
      • j decrements.

  3. Middle Plant: If i and j become equal, there's a middle plant.

    • If max(a, b) < plants[i], whoever has less water needs a refill before watering the middle plant (ans++).

  4. Return: The function returns ans, the total number of refills.

Time Complexity: O(n), where n is the number of plants. We iterate through the plants array once.

Space Complexity: O(1), as we use a constant number of variables regardless of the input size.

Code Examples (Python, Java, C++, Go, TypeScript, Rust):

The provided code snippets in various languages implement this two-pointer approach. They all follow the same logic described above, with minor syntactic differences based on the language. Each code snippet accurately reflects the steps outlined in the approach section. The comments within the code further clarify the logic at each step.