402. Remove K Digits

I was thinking the K digits must be consecutive but it turns out it's not necessary. So the problem becomes easier. All I need to do is to discard the number that is larger than current number. Line 13 is very trick. Without this line, the code fails the case of ["9", 1]

1:  class Solution {  
2:  public:  
3:    string removeKdigits(string num, int k) {  
4:      string res = "";  
5:      int n = num.size(), sz = n-k;  
6:      for (c : num) {  
7:        while (k && !res.empty() && res.back() > c) {  
8:          res.pop_back();  
9:          k--;  
10:        }  
11:        res += c;  
12:      }  
13:      res.resize(sz);  
14:      int i = 0;  
15:      while (!res.empty() && res[i] == '0') i++;  
16:      res = res.substr(i);  
17:      return res.empty() ? "0" : res;  
18:    }  
19:  };