Gas Station

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, otherwise return -1.


Note:

The solution is guaranteed to be unique.

一开始显然想到暴力破解

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int pgas, pcost, i, j, k;
        vector<int> passcost;
        for(i = 0; i < gas.size(); i++){
            passcost.push_back( gas[i] - cost[i]);
        }
        for(j = 0; j < gas.size(); j++){
            if(passcost[j] < 0) continue;
            pgas = passcost[j];
            for(k = 0; k< gas.size(); k++){
                pgas += passcost[((j+k)%gas.size())];
                if(pgas < 0) break;
            }
            if(k == gas.size()) return j;
        }
        return -1;
    }
};

果然 time limited exceed

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int pgas, pcost, i, j, k;
        vector<int> passcost;
        int sum = 0;
        for(i = 0; i < gas.size(); i++){
            passcost.push_back( gas[i] - cost[i]);
            sum += gas[i] - cost[i];
        }   
            
        if(sum < 0) return -1;
            
        int stMax=0, stmax=0, endMin=0;
        int min = passcost[0], MIN = 0, max = passcost[0], MAX= 0;
        for( i = 1; i < gas.size(); i++){
            if(max < 0){
                max = passcost[i];
                stmax = i;
            }
            else max+=passcost[i];
            
            if(max > MAX){
                MAX = max;
                stMax = stmax;
            }
            
            if(min > 0){
                min = passcost[i];
            }
            else min += passcost[i];
            
            if(min < MIN)
            {
                MIN = min;
                endMin = i;
            }
        }
        return (MAX >= (sum - MIN) ? stMax : (endMin+1) % gas.size());
    }
};

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int sum = 0, tem = 0, total = 0, i;
        for(i = 0; i < gas.size(); i++){
            total += gas[i] - cost[i];
            if(sum >= 0){
                sum += gas[i] - cost[i];
            }
            else{
                sum = gas[i] - cost[i];
                tem = i;
            }
        }   
        if(total < 0) return -1;
        else return tem;
    }
};