Leetcode 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: assume gas tank can go negative. We start from 0 to n-1. Monitor the gas tank level. If tank lowest happens after i before i+1, then i+1 should be the place of start point. It O(n) time O(1) space solution.

 public class Solution {  
   public int canCompleteCircuit(int[] gas, int[] cost) {  
     int min=Integer.MAX_VALUE, j=-1;  
     int res=0;  
     for (int i=0; i<gas.length; i++) {  
       res+=gas[i]-cost[i];  
       if (res<min) {  
         min=res;  
         j=i;  
       }  
     }  
     if (res<0) return -1;  
     if (j+1==gas.length) return 0;  
     return j+1;  
   }  
 }