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.

O(n^2)解法

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int n=gas.length;
        int x=0,steps=0;
        for(int i=0;i<n;i++){
            for(int j=i;j<i+n;j++){
                int k=j%n;
                x+=(gas[k]-cost[k]);
                if(x<0) {
                    steps=0;
                    x=0;
                    break;
                }
                else{
                    steps++;
                }
                if(steps==n && x>=0) return i;
            }
        }
        return -1;
    }
}

O(n)解法

The reason why I think this works: 1, if sum of gas is more than sum of cost, then there must be a solution. And the question guaranteed that the solution is unique(The first one I found is the right one). 2, The tank should never be negative, so restart whenever there is a negative number.

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