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99 lines
2.9 KiB
Python
99 lines
2.9 KiB
Python
"""
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Task:
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There are n gas stations along a circular route, where the amount of gas
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at the ith station is gas_quantities[i].
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You have a car with an unlimited gas tank and it costs costs[i] of gas
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to travel from the ith station to its next (i + 1)th station.
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You begin the journey with an empty tank at one of the gas stations.
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Given two integer arrays gas_quantities and costs, return the starting
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gas station's index if you can travel around the circuit once
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in the clockwise direction otherwise, return -1.
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If there exists a solution, it is guaranteed to be unique
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Reference: https://leetcode.com/problems/gas-station/description
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Implementation notes:
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First, check whether the total gas is enough to complete the journey. If not, return -1.
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However, if there is enough gas, it is guaranteed that there is a valid
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starting index to reach the end of the journey.
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Greedily calculate the net gain (gas_quantity - cost) at each station.
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If the net gain ever goes below 0 while iterating through the stations,
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start checking from the next station.
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"""
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from dataclasses import dataclass
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@dataclass
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class GasStation:
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gas_quantity: int
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cost: int
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def get_gas_stations(
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gas_quantities: list[int], costs: list[int]
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) -> tuple[GasStation, ...]:
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"""
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This function returns a tuple of gas stations.
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Args:
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gas_quantities: Amount of gas available at each station
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costs: The cost of gas required to move from one station to the next
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Returns:
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A tuple of gas stations
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>>> gas_stations = get_gas_stations([1, 2, 3, 4, 5], [3, 4, 5, 1, 2])
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>>> len(gas_stations)
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5
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>>> gas_stations[0]
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GasStation(gas_quantity=1, cost=3)
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>>> gas_stations[-1]
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GasStation(gas_quantity=5, cost=2)
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"""
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return tuple(
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GasStation(quantity, cost) for quantity, cost in zip(gas_quantities, costs)
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)
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def can_complete_journey(gas_stations: tuple[GasStation, ...]) -> int:
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"""
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This function returns the index from which to start the journey
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in order to reach the end.
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Args:
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gas_quantities [list]: Amount of gas available at each station
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cost [list]: The cost of gas required to move from one station to the next
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Returns:
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start [int]: start index needed to complete the journey
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Examples:
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>>> can_complete_journey(get_gas_stations([1, 2, 3, 4, 5], [3, 4, 5, 1, 2]))
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3
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>>> can_complete_journey(get_gas_stations([2, 3, 4], [3, 4, 3]))
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-1
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"""
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total_gas = sum(gas_station.gas_quantity for gas_station in gas_stations)
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total_cost = sum(gas_station.cost for gas_station in gas_stations)
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if total_gas < total_cost:
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return -1
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start = 0
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net = 0
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for i, gas_station in enumerate(gas_stations):
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net += gas_station.gas_quantity - gas_station.cost
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if net < 0:
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start = i + 1
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net = 0
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return start
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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