加油站

突破点：如果到某个加油站剩余油量不足0，就从下一个加油站重新开始。

class Solution {
public:
    /**
     *
     * @param gas int整型vector
     * @param cost int整型vector
     * @return int整型
     */
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        // write code here
        int size = gas.size();
        int sum = 0, current = 0, idx = -1;
        for (int i = 0; i < size; ++i) {
            sum += gas[i] - cost[i];
            current += gas[i] - cost[i];
            if (current < 0) {
                current = 0;
                idx = i;
            }
        }
        return sum >= 0 ? idx + 1 : -1;
    }
};