class Solution {
static size_t toNum(char c) {
    return c - '0';
}
public:
    string removeKdigits(string num, int k) {
    std::stack<char> st;

    for(const char& c : num) {
        while(k > 0 &&  !st.empty() && toNum(st.top()) > toNum(c)) {
            st.pop();
            --k;
        }

        st.push(c);
    }

   while(!st.empty() && k) {
      st.pop();
      --k;
   }

    std::string result;
    result.resize(st.size());
    size_t idx = st.size() - 1;

   while(!st.empty()) {
      char c = st.top();
       st.pop();
       result[idx--] = c;
   }
   idx = 0;
   while(idx < result.size() - 1 && result[idx] == '0') {++idx;}

   result = std::string(result.begin() + idx, result.end());


   return result == "" ? "0" : result;
}
};