Build the smallest.cpp

//{ Driver Code Starts
#include<bits/stdc++.h>
using namespace std;
string buildLowestNumber(string num, int k);
// Driver program to test above function
int main()
{
    int t,n;
    string str;
    cin>>t;
    while(t--)
    {
    cin>>n;
    cin>>str;
    cout << buildLowestNumber(str, n)<<endl;
    }
    return 0;
}

// } Driver Code Ends


string buildLowestNumber(string num, int k)
{
    stack<char>s;
    for(auto i : num){
        while(!s.empty() && s.top()>i && k>0) s.pop(),k--;
        s.push(i);
    }
    while(!s.empty() && k>0){
        s.pop();
        k--;
    }
    string ans="";
    while(!s.empty())ans+=s.top(),s.pop();
    while(!ans.empty() && ans[ans.length()-1]=='0') ans.pop_back();
    if(ans.length()==0) return "0";
    reverse(ans.begin(),ans.end());
    return ans;
     
}