class Solution {public:	vector<int> leftgas;	int len;    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {        len = gas.size();		int i=0;		leftgas.resize(gas.size());		for (i=0; i<len; i++)	    {         	leftgas[i] = gas[i] - cost[i];		}		pair<bool,int> temp;        temp = IndexComplete();        if(temp.first == true)		 return temp.second;		return -1;    }	pair<bool,int> IndexComplete()	{	  int allgas = 0;	  int backwardindex = 0;	  int forwardindex = 0;	  bool ret;      while(1)      {        allgas +=leftgas[forwardindex++];				if(allgas >= 0)		{		 		   if(forwardindex  == backwardindex || forwardindex  == len)		   	return pair<bool,int>(true,backwardindex);		}		else		{            ret = SupplyGas(allgas,backwardindex,forwardindex);			if(ret == false)			return pair<bool,int>(false,backwardindex);		}			  }	}	bool  SupplyGas(int &allgas,int& backwardindex,int forwardindex )	 {	  	        while(1)        {          if(backwardindex == 0)          {             backwardindex = len-1;		  }		  else		  {             backwardindex--;		  }		            allgas += leftgas[backwardindex];		  if(allgas >= 0)		  {             return true;		  }		  else		  {            if(backwardindex <= forwardindex)			  return false;		  }		 		 		}	 }};