Gas Station

Given two integer arrays A and B of size N.
There are N gas stations along a circular route, where the amount of gas at station i is A[i].

You have a car with an unlimited gas tank and it costs B[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 minimum starting gas station’s index if you can travel around the circuit once, otherwise return -1.

You can only travel in one direction. i to i+1, i+2, … n-1, 0, 1, 2.. Completing the circuit means starting at i and
ending up at i again.

The first argument given is the integer array A.
The second argument given is the integer array B.

Return the minimum starting gas station's index if you can travel around the circuit once, otherwise return -1.

Input 1:
    A =  [1, 2]
    B =  [2, 1]
Output 1:
    1
    Explanation 1:
        If you start from index 0, you can fill in A[0] = 1 amount of gas. Now your tank has 1 unit of gas. But you need B[0] = 2 gas to travel to station 1. 
        If you start from index 1, you can fill in A[1] = 2 amount of gas. Now your tank has 2 units of gas. You need B[1] = 1 gas to get to station 0. So, you travel to station 0 and still have 1 unit of gas left over. You fill in A[0] = 1 unit of additional gas, making your current gas = 2. It costs you B[0] = 2 to get to station 1, which you do and complete the circuit.

Method:

Start from index 0, we keep track of the amount of gas in tank, if tank < 0, then it means we cannot start from the current starting index, so we have to start from next, and we also keep track of the deficit. In the end, if tank >= deficit, then it means there is a solution.

Solution:

Time: O(n)
Space: O(1)

public class Solution {
    // DO NOT MODIFY THE LIST. IT IS READ ONLY
    public int canCompleteCircuit(final List<Integer> A, final List<Integer> B) {
        int tank = 0;
        int deficit = 0;
        int start = 0;
        for (int i = 0; i < A.size(); i ++) {
            tank += A.get(i) - B.get(i);
            if (tank < 0) {
                start = i + 1;
                deficit += tank;
                tank = 0;
            }
        }
        if (tank + deficit < 0 || start == A.size()) return -1;
        return start;
    }
}