lintcode-medium-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 costscost[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.

Given 4 gas stations with gas[i]=[1,1,3,1], and thecost[i]=[2,2,1,1]. The starting gas station‘s index is 2.

public class Solution {
    /**
     * @param gas: an array of integers
     * @param cost: an array of integers
     * @return: an integer
     */
    public int canCompleteCircuit(int[] gas, int[] cost) {
        // write your code here

        if(gas == null || gas.length == 0 || cost == null || cost.length == 0 || gas.length != cost.length)
            return -1;

        int gas_sum = 0;
        int cost_sum = 0;

        for(int i = 0; i < gas.length; i++)
            gas_sum += gas[i];

        for(int i = 0; i < cost.length; i++)
            cost_sum += cost[i];

        if(gas_sum < cost_sum)
            return -1;

        int left = 0;
        int start = 0;

        while(start < gas.length){
            int i = start;

            for(; i < gas.length; i++){
                left += gas[i];
                left -= cost[i];

                if(left < 0){
                    left = 0;
                    break;
                }
            }

            if(i != gas.length)
                start = i + 1;
            else
                return start;
        }

        return -1;
    }
}