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, otherwise return -1.

Note:

The solution is guaranteed to be unique.

Solution

若储油量总和sum(gas) >= 耗油量总和sum(cost)，则问题一定有解

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class Solution(object):
    def canCompleteCircuit(self, gas, cost):
        """
        :type gas: List[int]
        :type cost: List[int]
        :rtype: int
        """
        bal = 0
        start = 0
        for i in range(len(gas)):
            bal += gas[i] - cost[i]
            if bal < 0:
                start = i+1
                bal = 0
        if sum(gas) - sum(cost) >=0:
            return start
        else:
            return -1