leetcode学习笔记37

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 in the clockwise direction, otherwise return -1.

Note:

If there exists a solution, it is guaranteed to be unique.
Both input arrays are non-empty and have the same length.
Each element in the input arrays is a non-negative integer.
Example 1:

Input:
gas = [1,2,3,4,5]
cost = [3,4,5,1,2]

Output: 3

Explanation:
Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 4. Your tank = 4 - 1 + 5 = 8
Travel to station 0. Your tank = 8 - 2 + 1 = 7
Travel to station 1. Your tank = 7 - 3 + 2 = 6
Travel to station 2. Your tank = 6 - 4 + 3 = 5
Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
Therefore, return 3 as the starting index.
Example 2:

Input:
gas = [2,3,4]
cost = [3,4,3]

Output: -1

Explanation:
You can’t start at station 0 or 1, as there is not enough gas to travel to the next station.
Let’s start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
Travel to station 0. Your tank = 4 - 3 + 2 = 3
Travel to station 1. Your tank = 3 - 3 + 3 = 3
You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
Therefore, you can’t travel around the circuit once no matter where you start.

主要方法转自https://www.cnblogs.com/boring09/p/4248482.html
1、原始代码，无优化

class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        for(int i=0;i<cost.length;i++){
            if(gas[i]>=cost[i]){
                if(helper(gas,cost,i)){
                    return i;
                }
            }
        }
        return -1;
    }
    public boolean helper(int[] gas, int[] cost,int start) {
        int tank=gas[start];
        int len=cost.length;
        int cur=start,next=(cur+1)%len;
        while(next!=start){
            tank=tank-cost[cur]+gas[next];
            if(tank<cost[next])
                return false;
            cur=(cur+1)%len;
            next=(cur+1)%len;
        }
        if(tank>=cost[cur])
            return true;
        else
            return false;
    }
}

2、优化代码，时间复杂度为O（n),空间复杂度为O（1）

class Solution {
	public int canCompleteCircuit(int[] gas, int[] cost) {
		int start = 0; // 起始位置
		int remain = 0; // 当前剩余燃料
		int debt = 0; // 前面没能走完的路上欠的债

		for (int i = 0; i < gas.length; i++) {
			remain += gas[i] - cost[i];
			if (remain < 0) {
				debt += remain;
				start = i + 1;
				remain = 0;
			}
		}

		return remain + debt >= 0 ? start : -1;
	}

}