# 134. Gas Station (Medium)

https://leetcode.com/problems/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 in the clockwise direction, otherwise return -1.

Note:

• If there exists a solution, it is guaranteed to be unique.
• Both input arrays are non-empty and have the same length.
• Each element in the input arrays is a non-negative integer.

Example 1:

```Input:
gas  = [1,2,3,4,5]
cost = [3,4,5,1,2]

Output: 3

Explanation:
Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 4. Your tank = 4 - 1 + 5 = 8
Travel to station 0. Your tank = 8 - 2 + 1 = 7
Travel to station 1. Your tank = 7 - 3 + 2 = 6
Travel to station 2. Your tank = 6 - 4 + 3 = 5
Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
Therefore, return 3 as the starting index.
```

Example 2:

```Input:
gas  = [2,3,4]
cost = [3,4,3]

Output: -1

Explanation:
You can't start at station 0 or 1, as there is not enough gas to travel to the next station.
Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 0. Your tank = 4 - 3 + 2 = 3
Travel to station 1. Your tank = 3 - 3 + 3 = 3
You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
Therefore, you can't travel around the circuit once no matter where you start.
```

## Solutions

``````class Solution {

public int canCompleteCircuit(int[] gas, int[] cost) {
if (gas == null || cost == null || gas.length == 0 || cost.length == 0) {
return -1;
}

int len = gas.length;

int gasSum = 0;
int costSum = 0;
for (int i = 0; i < len; i++) {
gasSum += gas[i];
costSum += cost[i];
}

// We can sure we are not able to travel around the circuit if the total gas that costs
// more than it supplied.
if (gasSum < costSum) {
return -1;
}

// the initial start point
int start = 0;

while (true) {

// Gas you get when travelling
gasSum = 0;

// Gas you cost when travelling
costSum = 0;

int i;
for (i = 0; i < len; i++) {
int newI = start + i;
if (newI >= len) {
newI = (start + i) % len;
}

// you carry on from gas station newI and add gas[newI]
gasSum += gas[newI];

// you drive to gas station (newI + 1) / len
costSum += cost[newI];

// you have to make sure the gas suffices for the later journey.

// if cost > supplement, you need to change the start point next to newI
if (costSum > gasSum) {
start = (newI + 1) % len;

break;
}
}

if (i == len) {
break;
}
}

// There will always be a proper start point
return start;
}
}
``````