[LeetCode] Gas Station

Problem:

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:O(n)

public class Solution{
   public int canCompleteCircuit(int[] gas, int[] cost) {
      int num = gas.length;
      int seqStart = 0;
      int max = 0;
      for (int i = 0; i < 2 * num; ++i) {

         int index = i % num;

         int remain = gas[index] - cost[index];
         if (max < 0) {
            max = remain;
            seqStart = i;

         } else {

            max = max + remain;
         }
      }

      if (seqStart >= num) {

         return -1;
      }
      return seqStart;
   }
}