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.

Analysis

• total用来记录所有的gas - cost
• If car starts at A and can not reach B. Any station between A and B can not reach B.(B is the first station that A can not reach.)
• 一旦tank < 0，start就可以记为i+1，tank重新记为0

Code

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int start=0, total=0, tank=0;
        //if car fails at 'start', record the next station
        for(int i=0;i<gas.length;i++) {
            tank = tank+gas[i]-cost[i];
            if(tank<0) {
               start=i+1;
               total+=tank;
               tank=0;
            }
        }
        return (total+tank<0)? -1:start;
    }
}

Reference

Leetcode