Remove K Digits

// C++ program to build the smallest number by removing n digits from 
// a given number 
#include<iostream> 
using namespace std; 
  
// A recursive function that removes 'n' characters from 'str' 
// to store the smallest possible number in 'res' 
void buildLowestNumberRec(string str, int n, string &res) 
{ 
    // If there are 0 characters to remove from str, 
    // append everything to result 
    if (n == 0) 
    { 
        res.append(str); 
        return; 
    } 
  
    int len = str.length(); 
  
    // If there are more characters to remove than string 
    // length, then append nothing to result 
    if (len <= n) 
        return; 
  
    // Find the smallest character among first (n+1) characters 
    // of str. 
    int minIndex = 0; 
    for (int i = 1; i<=n ; i++) 
        if (str[i] < str[minIndex]) 
            minIndex = i; 
  
    // Append the smallest character to result 
    res.push_back(str[minIndex]); 
  
    // substring starting from minIndex+1 to str.length() - 1. 
    string new_str = str.substr(minIndex+1, len-minIndex); 
  
    // Recur for the above substring and n equals to n-minIndex 
    buildLowestNumberRec(new_str, n-minIndex, res); 
} 
  
// A wrapper over buildLowestNumberRec() 
string buildLowestNumber(string str, int n) 
{ 
    string res = ""; 
  
    // Note that result is passed by reference 
    buildLowestNumberRec(str, n, res); 
  
    return res; 
} 
  
// Driver program to test above function 
int main() 
{ 
    string str = "121198"; 
    int n = 2; 
    cout << buildLowestNumber(str, n); 
    return 0; 
}