Gas Station

Question

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.

Example Given 4 gas stations with gas[i]=[1,1,3,1], and the cost[i]=[2,2,1,1]. The starting gas station's index is 2.

Note The solution is guaranteed to be unique.

Challenge O(n) time and O(1) extra space

Thoughts

首先，如果总的gas小于总的cost, 一定走不到。 如果大于等于，说明一定有解

然后到了每站剩下的油加上该站的油gas[i]也到不了下一站的话，就讲下一站设为起点

reference

Solution

class Solution:
    # @param gas, a list of integers
    # @param cost, a list of integers
    # @return an integer
    def canCompleteCircuit(self, gas, cost):
        # write your code here
        if sum(gas) < sum(cost):
            return - 1
        n = len(gas)
        #到第一站并没有消耗
        diff = 0
        stationIndex = 0
        for i in range(n):
            if gas[i] + diff < cost[i]:
                #将下一站设为起点
                stationIndex = i + 1
                diff = 0
            else:
                #剩下的油是累加的
                diff += gas[i] - cost[i]

        return stationIndex