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

        int sum = 0;
        int total = 0;
        int index = -1;

        for(int i = 0; i<gas.length; i++) {
            sum += gas[i] - cost[i];
            total += gas[i] - cost[i];
            if(sum < 0) {
                index = i;
                sum = 0;
            }
        }
        return total < 0 ? -1 : index + 1;
        // index should be updated here for cases ([5], [4]);
        // total < 0 is for case [2], [2]
    }
}

class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int currentNetSum = 0;
        int startIdx = -1;
        int totalNetDiff = 0;
        
        for(int idx = 0; idx < gas.length; idx++) {
            int currentDiff = gas[idx] - cost[idx];
            currentNetSum += currentDiff;
            totalNetDiff += currentDiff;
            if(currentNetSum < 0) {
                currentNetSum = 0;
                startIdx = -1;
                continue;
            }
            
            startIdx = startIdx == -1 ? idx: startIdx;
        }
        
        return startIdx != -1 && totalNetDiff >= 0 ? startIdx: -1;
    }
}