• Time:O(n)
• Space:O(1)

## C++

``````class Solution {
public:
int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
const int gasses = accumulate(begin(gas), end(gas), 0);
const int costs = accumulate(begin(cost), end(cost), 0);
if (gasses - costs < 0)
return -1;

int ans = 0;
int sum = 0;

// try to start from each index
for (int i = 0; i < gas.size(); ++i) {
sum += gas[i] - cost[i];
if (sum < 0) {
sum = 0;
ans = i + 1;  // start from next index
}
}

return ans;
}
};
``````

## JAVA

``````class Solution {
public int canCompleteCircuit(int[] gas, int[] cost) {
final int gasses = Arrays.stream(gas).sum();
final int costs = Arrays.stream(cost).sum();
if (gasses - costs < 0)
return -1;

int ans = 0;
int sum = 0;

// try to start from each index
for (int i = 0; i < gas.length; ++i) {
sum += gas[i] - cost[i];
if (sum < 0) {
sum = 0;
ans = i + 1; // start from next index
}
}

return ans;
}
}
``````

## Python

``````class Solution:
def canCompleteCircuit(self, gas: List[int], cost: List[int]) -> int:
ans = 0
net = 0
sum = 0

for i in range(len(gas)):
net += gas[i] - cost[i]
sum += gas[i] - cost[i]
if sum < 0:
sum = 0
ans = i + 1

return -1 if net < 0 else ans
``````