int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {

// Note: The Solution object is instantiated only once and is reused by each test case.

vector<int> diff;

int size = gas.size();

if(size == 0)

return -1;

for(int i = 0; i < size; ++i)

diff.push_back(gas[i] - cost[i]);

for(int i = 0; i < size; ++i)

diff.push_back(diff[i]);

int left = 0, right = 0;

int sum = 0;

while(left < size)

{

while(right < 2 * size && sum >= 0)

{

if(right - left == size)

return left;

sum += diff[right];

++right;

}

while(left < size && left < right && sum < 0)

{

sum -= diff[left];

++left;

}

}

return -1;

}