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.

把路径上所有值加起来，找到第一个，从该点到整个路径的和大于0的下标；如果不存在这样的点， 就输出-1；

int canCompleteCircuit(int* gas, int gasSize, int* cost, int costSize) {
    int index = 0;
    int suc_sum = 0;
    int add_sum = 0;
    
    // go over the gas[] and cost[]
    for(int i = 0; i < gasSize; i++){
        int j = gas[i] - cost[i];
        // record add sum and the first Non-negative index and its successive sum 
        add_sum += j;
        suc_sum += j;
        if(suc_sum  < 0){
            index = i + 1;
            suc_sum = 0;
        }
    }
    // if sum is Non-negative return the first Non-negatice number
    if(add_sum >= 0){
        return index;
    }else{
        // else return -1
        return -1;
    }
    
}