Remove K Digits in C++

Suppose we have a non-negative integer num that is represented as a string, we have to remove k digits from the number so that the new number is the smallest possible. So if the input is like “1432219” and k = 3, then the result will be “1219”.

To solve this, we will follow these steps −

• Define a stack st, create an empty string ret

• n := size of num

• for i in range 0 to n – 1

  • while k is non zero and stack is not empty and top of stack > num[i]

    • delete from the stack and decrease k by 1

  • insert num[i] into st

• while k is not 0, delete element from the stack

• while the stack is not empty

  • ret := ret + top of stack, delete element from stack

• now reverse the ret string

• ans := an empty string, and i := 0

• while i < size of ret and ret[i] is not ‘0’

  • increase i by 1

• for i < size of ret

  • ans := ans + ret[i]

• ret := ans

• return “0” if size of ret is 0, otherwise, ret

Example (C++)

Let us see the following implementation to get a better understanding −

class Solution {
public:
   string removeKdigits(string num, int k) {
      stack st;
      string ret = "";
      int n = num.size();
      for(int i = 0; i < n; i++){
         while(k && !st.empty() && st.top() > num[i]){
            st.pop();
            k--;
         }
         st.push(num[i]);
      }
      while(k--)st.pop();
      while(!st.empty()){
         ret += st.top();
         st.pop();
      }
      reverse(ret.begin(), ret.end());
      string ans = "";
      int i = 0;
      while(i <ret.size() && ret[i] == '0')i++;
      for(; i < ret.size(); i++)ans += ret[i];
      ret = ans;
      return ret.size() == 0? "0" : ret;
   }
};

Input

"1432219"
3

Output

"1219"