[LeetCode] - Gas Station

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.
这个题目的关键就是得想通一个问题，如果station[i...j]的耗油量小于补给量，那么i到j之间，也包括i和j的所有点都是不能作为起点的。这有点儿类似于maximum subarray的感觉。也就是说，如果一个区间的和小于零，再将这个和减去一个必然是正数的部分，这个和只能更小于零了。有点儿绕，需要仔细想清楚才行。这样做，就可以排除重复的check，达到线性的复杂的。
代码如下：

public class Solution {
public int canCompleteCircuit(int[] gas, int[] cost) {
if(gas==null || cost==null) return -1;
int i = 0;
while(i < gas.length) {
int gasLeft=gas[i]-cost[i], j=i+1;
while(gasLeft >= 0) {
if(j%gas.length==i) return i;
gasLeft += gas[j%gas.length]-cost[j%gas.length];
j++;
}
i = j;
}
return -1;
}
}