Plus One

Given a non-negative number represented as an array of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

用carrier表示进位，从尾部开始逐位进行加法和进位计算，直到digits的index到0并且carrier为0同时满足。 最后反转存放结果的vector，因为我们是从小头开始挨个插入的。

class Solution {
public:
    vector<int> plusOne(vector<int> &digits) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int carrier = 1;
        vector<int> ret;
        auto it = back_inserter(ret);
        for (int i = digits.size() - 1; i >= 0 || carrier > 0;)  {
            int num = (i >= 0 ? digits[i--] : 0) + carrier;
            carrier = num / 10;
            it = num % 10;
        }
        
        reverse(ret.begin(), ret.end());
        return ret;
    }
};