Gas Station

题目描述：
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.

分析：
考虑从每个点出发的可行性即可

代码：

int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
    if(gas.size()==1) {
        if(gas[0]>=cost[0]) return 0;
        return -1;
      }
        for(int i=0;i<gas.size();i++){
            int num=0;
            int ok=0;
            int j;
            for( j=0;j<gas.size();j++){
            int s=i+j;
            if(s>=gas.size()) s=s%gas.size();
                num+=gas[s];
                num-=cost[s];
                if(num>=0){
                ok++;
                if(ok==gas.size()) return i;
                }
                else{
                    break;
                }
            }


        }
        return -1;
    }