Saturday, May 13, 2017

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.



Solution:

1. If from start we cannot get to station i, we know from start, no one can get to station i. Therefore station 0 to i can be safely removed from candidates.

2. If from to end, the total gas - total cost < 0, we know it is impossible from any station to drive a loop. So there is no solution.



Code:


public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int totalGas = 0;
        int leftOver = 0;
        int res = 0;
        for (int i = 0; i < gas.length; i++) {
            totalGas += gas[i] - cost[i];
            leftOver += gas[i] - cost[i];
            if (leftOver < 0) {
                leftOver = 0;
                res = i + 1;
            }
        }
        return totalGas >= 0 ? res : -1;
    }
}