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NeetCode #134LC-1603EasyArrays & Hashing
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#134 · #1603 · Design Parking System(设计停车系统)

📌 Problem Statement & Constraints

Design a parking system with big, medium and small slots. Implement addCar(carType) returning true if a slot of that size is free (and occupying it), or false otherwise. Constraints: 0 <= big, medium, small <= 1000, 1 <= carType <= 3; at most 1000 calls.

💡 Core Algorithmic Approaches

  1. Keep three counters, one per slot size.
  2. Index them directly by carType - 1 or store a length-4 array so carType maps straight to the slot.
  3. On addCar, decrement the counter if positive and return true; otherwise return false.
  4. This is a pure state-machine warm-up; the only care needed is not to decrement when full.

💻 Benchmark Python3 Implementation

class ParkingSystem:
    def __init__(self, big: int, medium: int, small: int):
        self.slots = [0, big, medium, small]   # index by carType directly

    def addCar(self, carType: int) -> bool:
        if self.slots[carType] > 0:
            self.slots[carType] -= 1
            return True
        return False

⚡ Complexity Deep Dive

⏱️ Time Complexity
O(1) per call.
💾 Space Complexity
O(1): four integers.

⚠️ Interview Pitfalls & Follow-ups

  • Decrementing below zero: the > 0 guard is required, and the counter must not go negative.
  • Using carType - 1 with a length-3 list: both work, but mixing the two conventions is a classic source of off-by-one bugs. Pick one.
  • Returning the remaining count instead of a boolean: the API returns a boolean.