I found that my wrong code was accepted.

```
class Solution {
public:
string removeKdigits(string num, int k) {
if (k == 0)
return num;
int len = num.size();
string ret;
ret.push_back(num[0]);
for (int i = 1; i < len; i++) {
while (ret.size() != 0 && ret.back() > num[i] && k-- > 0) {
ret.pop_back();
}
if (ret.size() != 0 || num[i] != '0') {
ret.push_back(num[i]);
}
}
ret.resize(len - k);//there shoud minus the origin k
if (ret.size() == 0) {
return "0";
}
return ret;
}
};
```