Leetcode @ 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:

in order to continue the journey, we have to make sure that after each station, the gas should be no less than 0; if the total value is larger or equal to 0; there must be one place from where the sum of gas minus cost is always larger or equal to 0;

So in order to find this place we need to use the greddy method, if the sum is less than 0; we just ignore the position, becasue from this place there would be a situation that the sum of gas is less than 0, but because there must be one place and we can find
one. And we use the total value to judge whether or not there exists a value.

public class Solution {
public int canCompleteCircuit(int[] gas, int[] cost) {
int length = cost.length;
int sum = 0;
int start = 0;
total = 0
for(int i = 0; i < length; i++){
sum += (gas[i] - cost[i]);
total += (gas[i] - cost[i]);
if(sum < 0){
sum = 0;
start = i + 1;
}
}
if(total >= 0)
return start;
else
return -1;
}