代码：

/**
 * @param {number[]} gas
 * @param {number[]} cost
 * @return {number}
 */
var canCompleteCircuit = function(gas, cost) {
    var arr = [];
    var curSum = 0;
    var positiveSum = 0;
    var positiveIndex = 0;
    var index = gas.length - 1;
    gas.forEach((item, index) => {
        arr.push(item - cost[index])
    });
    while (index >= 0) {
        if (arr[index] < 0) {
            curSum = arr[index--];
            while (curSum < 0 && index >= 0) {
                curSum += arr[index--];
            }
            if (index < 0) {
                if (curSum + positiveSum >= 0) {
                    return positiveIndex
                }
                else {
                    return -1
                }
            }
            else {
                positiveSum += curSum;
                positiveIndex = index + 1;
                curSum = 0;
            }
        }
        else {
            while (arr[index] >= 0 && index >= 0) {
                positiveSum += arr[index];
                positiveIndex = index--;
            }
            if (index < 0) {
                return 0
            }
        }
    }
    return -1
};