**Description**Hints**Submissions**Solutions

Total Accepted: 83932 Total Submissions: 289062 Difficulty: Medium Contributor: LeetCode

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.

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

// O(1) space O(n) running time solution
class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {

       int start = gas.size()-1;
       int end = 0;
       int sum = gas[start] - cost[start];
       while (start > end) {
          if (sum >= 0) {
             sum += gas[end] - cost[end];
             ++end;
          }
          else {
             --start;
             sum += gas[start] - cost[start];
          }
       }
       return sum >= 0 ? start : -1;
    }
};