134. Gas Station

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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.

解法1： Greedy

lang: java
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class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        if (gas == null || gas.length == 0 || cost == null || cost.length == 0) {
            return -1;
        }
        int sum = 0, total = 0, pos = -1;
        for (int i = 0; i < gas.length; i++) {
            sum += gas[i] - cost[i];
            total += sum;
            if (sum < 0) {
                pos = i;
                sum = 0;    // reset sum to be zero
            }
        }
        if (total < 0) {
            return -1;
        }
        return pos + 1;
    }
}