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.

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

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

}
};