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.

最大子数组和的应用    时间复杂度为O(n)

class Solution {
public:
int canCompleteCircuit(vector<int> &gas, vector<int> &cost)
{
int n = gas.size();
int sum = 0;
int total = 0;
int startindex = 0;
for(int i = 0; i < n; i++)
{
total = total + gas[i] - cost[i];
sum = sum + gas[i] - cost[i];
if(sum < 0)
{
startindex = i + 1;
sum = 0;
}
}
if (total < 0)
return -1;
else
return startindex;

}
};