leetCode(54):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.

class Solution {public:    int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {        int length=gas.size();    vector<int> tmpGas(length,0);        for(int i=0;i<length;++i)    {    tmpGas[i]=gas[i]-cost[i];//两者之差组成的新数组，代表补充或消耗    }        int startIndex=0;//初始起点    int sum=tmpGas[startIndex];        int i=0;    while(true)    {    i++;    if(i>=length)    {    i=i-length;    }        if(sum<0)    {    startIndex=i;//以下一个节点作为起点    if(startIndex==0)    return -1;//又回到初始起点，开始出现循环    sum=tmpGas[startIndex];    }    else    {    sum+=tmpGas[i];

<span style="white-space:pre"></span>    //已经到达终点，判断是否能够顺利开完全程    if(startIndex==0 && i==length-1)    {//如果起点是0，当前结点又是最后一个节点    if(sum<0)    return -1;    else    return startIndex;    }        if(i==startIndex-1)    {//如果当前结点是循环的最后一个结点    if(sum>=0)    return startIndex;    else    return -1;    }    }    }    }};