题目：
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.
题解：
网上搜到的解法。
据说是Bloomberg的面试题。
用两个变量存+= gas[i] - cost[i]。一个帮助判断当前这个点作为gas station的起点合不合适，一个帮助判断总的需求是不是大于供给。如果总的需求大于供给那么肯定是无解的，如果需求小于等于供给，就可以返回刚才找到的起始点。
代码如下：
1 public int canCompleteCircuit(int[] gas, int[] cost) {
2 if (gas==null|| cost==null||gas.length==0||cost.length==0||gas.length!=cost.length)
3 return -1;
4
5 int sum = 0;
6 int total = 0;
7 int index = 0;
8 for(int i = 0; i < gas.length; i++){
9 sum += gas[i]-cost[i];
10 total += gas[i]-cost[i];
11 if(sum < 0){
12 index=i+1;
13 sum = 0;
14 }
15 }
16 if(total<0)
17 return -1;
18 else
19 return index;
20 }
2 if (gas==null|| cost==null||gas.length==0||cost.length==0||gas.length!=cost.length)
3 return -1;
4
5 int sum = 0;
6 int total = 0;
7 int index = 0;
8 for(int i = 0; i < gas.length; i++){
9 sum += gas[i]-cost[i];
10 total += gas[i]-cost[i];
11 if(sum < 0){
12 index=i+1;
13 sum = 0;
14 }
15 }
16 if(total<0)
17 return -1;
18 else
19 return index;
20 }
Reference：http://blog.csdn.net/lbyxiafei/article/details/12183461