根据传入的数字，把不同位数一直相乘，直到结果小于10为止返回运算次数

var mount = 0;
function persistence(num) {
    var numArr = num.toString().split("");
    var total = 1;
    for (var i = 0; i < numArr.length; i++) {
        total *= parseInt(numArr[i]);
    }
    if (numArr.length !== 1) {
        mount++;
    } else {
        return 0;
    }
    if (total < 10) {
        return mount;
    } else {
        return persistence(total);
    }
}