[LeetCode]134. Gas Station

134. Gas Station

here 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.

分析

题目要求只能遍历所有加油站一遍后需要找出能完成整个行程的出发站点。那就从第一个站点开始遍历，用一个sum累计

gas[i]-cost[i]

,如果当前累计sum<0;说明从第一个站点前面任一个站点出发都不能完成行程，那就从当前站点重新累计。同时一路用一个total累计所有

gas[i]-cost[i]

，total最后的值来判断是否可以完成所有行程。

源码

class Solution {
public:
//O(n)的解法是遍历所有站点，用sum记录当前选择的出发点的有效性
// 用total来判断整个数组是否有解，有解的话返回sum所在下标
int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
int sum = 0;
int total = 0;
int start_index = -1;
for(int i = 0; i < gas.size(); i++) {
sum += gas[i] - cost[i];
total += gas[i] - cost[i];

if(sum < 0) {
sum = 0;
start_index = i;
}
}
return total >= 0 ? start_index + 1 : -1;
}
};