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Question

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

Result: Accepted Time: 0 ms

Here should be some explanations.

int canCompleteCircuit(int* gas, int gsz, int* cost, int csz) {
    for(int start = 0; start < csz;start++)
    {
        int tank =  gas[start]  - cost[start],now = (start + 1)%gsz,stop = start;
        while(tank >= 0 && now != stop)
        {
            tank += gas[now]  - cost[now];
            now++,start++;
            now %= gsz;
        }
        if(tank >= 0 && now == stop)
            return stop;
        if(stop!= start) start--;
    }
    return -1;
}

Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$