leetcode 66(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.
例如：数组{1,2,3}，加一以后为{1,2,4}。数组{9}加一以后为{1,0}。
方法一：开辟新的空间。

vector<int> plusOne(vector<int>& digits)
{
vector<int> res;
if (digits.empty())
res.push_back(1);
res.resize(digits.size());
int flag = 1;
for (int i = digits.size() - 1; i >= 0; --i)
{
int val = digits[i] + flag;
if (val >= 10)
{
res[i] = val - 10;
}
else
{
flag = 0;
res[i] = val;
}
}
if (flag)
{
res.push_back(0);
for(int i = res.size() - 1; i > 0; --i)
{
res[i] = res[i - 1];
}
res[0] = 1;
}
return res;
}

vector<int> plusOne(vector<int>& digits)
{
int len = digits.size();
for(int i = len - 1; i >= 0; --i)
{
if(digits[i] == 9)
{
digits[i] = 0;
}
else
{
digits[i]++;
return digits;
}
}
digits[0] = 1;
digits.push_back(0);
return digits;
}