gas-station.py

# -*- coding:utf-8 -*-


# There are N gas stations along a circular route, where the amount of gas at station i is gas[i].
#
# You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from station i to its next station (i+1). You begin the journey with an empty tank at one of the gas stations.
#
# Return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1.
#
# Note:
#
#
# 	If there exists a solution, it is guaranteed to be unique.
# 	Both input arrays are non-empty and have the same length.
# 	Each element in the input arrays is a non-negative integer.
#
#
# Example 1:
#
#
# Input: 
# gas  = [1,2,3,4,5]
# cost = [3,4,5,1,2]
#
# Output: 3
#
# Explanation:
# Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
# Travel to station 4. Your tank = 4 - 1 + 5 = 8
# Travel to station 0. Your tank = 8 - 2 + 1 = 7
# Travel to station 1. Your tank = 7 - 3 + 2 = 6
# Travel to station 2. Your tank = 6 - 4 + 3 = 5
# Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
# Therefore, return 3 as the starting index.
#
#
# Example 2:
#
#
# Input: 
# gas  = [2,3,4]
# cost = [3,4,3]
#
# Output: -1
#
# Explanation:
# You can't start at station 0 or 1, as there is not enough gas to travel to the next station.
# Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
# Travel to station 0. Your tank = 4 - 3 + 2 = 3
# Travel to station 1. Your tank = 3 - 3 + 3 = 3
# You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
# Therefore, you can't travel around the circuit once no matter where you start.
#
#


class Solution(object):
    def canCompleteCircuit(self, gas, cost):
        """
        :type gas: List[int]
        :type cost: List[int]
        :rtype: int
        """
        if len(gas) == 0 or len(cost) == 0 or sum(gas) < sum(cost):
            return -1
        position = 0
        balance = 0 # current tank balance
        for i in range(len(gas)):
            balance += gas[i] - cost[i] # update balance
            if balance < 0: # balance drops to negative, reset the start position
                balance = 0
                position = i+1
        return position