add gas station

greedy_methods/gas_station.py

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