假设a和b均为正数，且a表示的数比b大

vector<int> minus(vector<int> a, vector<int> b) {
vector<int> c;
int len1 = a.size();
int len2 = b.size();
int i, t;

for (i = 0; i < len2; i++) {
if (a[i] < b[i]) { //借位
____ // 在此处填入代码
a[i] += 10;
}
t = a[i] - b[i];
c.push_back(t);
}
for (; i < len1; i++)
c.push_back(a[i]);

len3 = c.size();
while (c[len3 - 1] == 0) { //去除前导0
c.pop_back();
len3--;
}
return c;
}