text: /*
  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.
  */
  
  #include <vector>
  #include <iostream>
  
  class Solution {
  public:
      int canCompleteCircuit(std::vector<int>& gas, std::vector<int>& cost) {
          size_t len = gas.size();
          int diff = 0;
          int start = 0;
          int sum = 0;
          for (int i = 0; i < len; ++i) {
              diff += gas[i] - cost[i];
              if (sum < 0) {
                  start = i; 
                  sum = gas[i] - cost[i];
              } else {
                  sum += gas[i] - cost[i];
              }
          }
          return diff < 0 ? -1 : start;
      }
  };
  
  