LeetCode [134] Gas Station

134. Gas Station

Medium

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.

 

Note

• start: starting gas station
• reach: reachable position from start
• rem: remaining gas
• increase reach if rem >=0; otherwise decrease start to look for more gas at earlier stations. Return start if reach is equal to start or -1 if all stations have been checked as start.
• Specifically,
• if rem>=0,
• check reach+1 station (clockwise direction)
• update rem accordingly
• otherwise, cannot arrive at reach from start. 
• check start-1 station (counter clockwise direction)
• update rem accordingly
• if reach == start, return start
• firstTimeBack prevents infinite iterations

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

class Solution { public: int canCompleteCircuit(vector<int>& gas, vector<int>& cost) { int n = gas.size(); int start = 0, reach = 0; int rem = gas[reach]-cost[reach]; bool firstTimeBack = true; while(true){ if(rem>=0){ reach = (reach+1)%n; if(reach==start) return start; rem += gas[reach]-cost[reach]; }else{ if(firstTimeBack){ start = n-1; firstTimeBack = false; }else if(start==0){ return -1; }else{ start--; } rem += gas[start]-cost[start]; } } } };

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

class Solution { public: int canCompleteCircuit(vector<int>& gas, vector<int>& cost) { int n = gas.size(); int start = 0, end = 0, rem = 0; while(true){ if(rem+gas[end]>=cost[end]){ rem += gas[end]-cost[end]; end++; if(end%n==start) return start; }else{ if(start==0) start = n-1; else start--; if(start==end) return -1; rem += gas[start]-cost[start]; } } } };