LC402.cpp

/*
402. Remove K Digits
Given a non-negative integer num represented as a string, remove k digits from the number 
so that the new number is the smallest possible.

Note:
The length of num is less than 10002 and will be ≥ k.
The given num does not contain any leading zero.
Example 1:

Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
Example 2:

Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.
Example 3:

Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.
*/

class Solution {
public:
    Solution()
    {
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);
    }
    string removeKdigits(string num, int k) {
        string strStack="";
        
        for(auto& elem : num)
        {
            while(strStack.size() && strStack.back() >elem && k)
            {
                strStack.pop_back();
                k--;
            }
            if(strStack.size()|| elem != '0') 
                strStack.push_back(elem);
        }
        
        while(strStack.size() && k--)
        {
            strStack.pop_back();
        }
        return (strStack == "")? "0" : strStack;
    }
};