Lowest Number by Removing k digits

#include <bits/stdc++.h>
using namespace std;

string lowestNum(string s, int k) {
    
    // Base Case 1: If k == 0, return the whole 's'
    if (k == 0) {
        return s;
    }

    int n = s.size();

    // Base Case 2: If 'n' is smaller 
    // or equal to k, everything can be removed
    if (n <= k) {
        return "";
    }

    // Find the smallest character among 
    // the first (k+1) characters of 's'
    int minIdx = 0;
    for (int i = 1; i <= k; ++i) {
        if (s[i] < s[minIdx]) {
            minIdx = i;
        }
    }

    // Append 's[minIdx]' and recur for the substring after minIdx
    return s[minIdx] + lowestNum(s.substr(minIdx + 1), k - minIdx);
}

string removeK(string s) {
    int i = 0;
    while (i < s.size() && s[i] == '0') {
        i++;
    }
    return i == s.size() ? "0" : s.substr(i);
}

int main() {
    string s = "765028321";
    int k = 5;
    string res = lowestNum(s, k);
    cout << removeK(res) << "\n";
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

void sortedInsert(deque<char>& dq, char ch)
{
    // If deque is empty, simply push the character
    if (dq.empty()) {
        dq.push_back(ch);
    } else {
        char temp = dq.back();

        // Remove digits from the back of deque
        // that are larger than the current character
        while (temp > ch && !dq.empty()) {
            dq.pop_back();
            if (!dq.empty())
                temp = dq.back();
        }

        // Insert the current char at back in the deque
        dq.push_back(ch);
    }
}

// Function to build the smallest number by removing 'k' digits
string lowestNum(string s, int k)
{
    int n = s.length();
    
    int digitsToKeep = n - k; 
    
    // Deque to store digits in non-decreasing order
    deque<char> dq;   
    string res = ""; 

    // Iterate through the first (n - k) characters of 's'
    int i;
    for (i = 0; i <= n - digitsToKeep; i++) {
        sortedInsert(dq, s[i]); 
    }

    // Process the remaining characters
    while (i < n) {
        
        // Add the smallest digit to the result string
        res += dq.front();  
        dq.pop_front();     
        
        // Insert the next character into 
        // the deque maintaining order
        sortedInsert(dq, s[i]);   
        i++;
    }

    // Add the last remaining digit in the deque to the result
    res += dq.front();
    dq.pop_front();
    return res;
}

// Function to remove leading zeros from the final result
string removeLe(string s, int k)
{
    string res = lowestNum(s, k);  
    string ans = "";               
    int flag = 0;         
    
    // Traverse through the result string 
    // and remove leading zeros
    for (int i = 0; i < res.length(); i++) {
        if (res[i] != '0' || flag == 1) { 
            flag = 1;
            ans += res[i]; 
        }
    }

    // If the result string is empty, return "0", 
    // else return the result string
    return ans.empty() ? "0" : ans;
}

int main()
{
    string s = "765028321"; 
    int k = 5;               
    cout << removeLe(s, k) << endl;
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

string remKdig(string s, int k)
{
    int n = s.size();
    stack<char> stk;
    
    for (char c : s) {
        while (!stk.empty() && k > 0 && stk.top() > c) {
            stk.pop();
            k -= 1;
        }

        if (!stk.empty() || c != '0')
            stk.push(c);
    }

    while (!stk.empty() && k--)
        stk.pop();
    
    if (stk.empty())
        return "0";

    while (!stk.empty()) {
        s[n - 1] = stk.top();
        stk.pop();
        n -= 1;
    }
    
    return s.substr(n);
}

int main()
{
    string s = "765028321";
    int k = 5;
    cout << remKdig(s, k);
    return 0;
}