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"