Gas Station 解答

Problem

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

Key to the solution is a conclusion:

If sum of gas >= sum of costs, then there must exists one or more solution.

If sum of gas < sum of costs, then there is no solution.

So we can use method of exclusion here.

We need also note here that if A can not reach C in a the sequence of A-->B-->C, then B can not make it either.

public class Solution {
public int canCompleteCircuit(int[] gas, int[] cost) {
if (gas == null || cost == null)
return -1;
if (gas.length != cost.length)
return -1;
int start = 0;
int sumRemaining = 0;
int totalRemaining = 0;
for (int i = 0; i < gas.length; i++) {
int tmp = gas[i] - cost[i];
if (sumRemaining >= 0) {
sumRemaining += tmp;
} else {
start = i;
sumRemaining = tmp;
}
totalRemaining += tmp;
}
if (totalRemaining < 0)
return -1;
return start;
}
}