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# Build Lowest Number by Removing n digits from a given number

Given a string ‘str’ of digits and an integer ‘n’, build the lowest possible number by removing ‘n’ digits from the string and not changing the order of input digits.

Examples:

```Input: str = "4325043", n = 3
Output: "2043"

Input: str = "765028321", n = 5
Output: "0221"

Input: str = "121198", n = 2
Output: "1118"```
Recommended Practice

The idea is based on the fact that a character among first (n+1) characters must be there in resultant number. So we pick the smallest of first (n+1) digits and put it in result, and recur for the remaining characters. Below is complete algorithm.

```Initialize result as empty string
res = ""
buildLowestNumber(str, n, res)
1) If n == 0, then there is nothing to remove.
Append the whole 'str' to 'res' and return

2) Let 'len' be length of 'str'. If 'len' is smaller or equal
to n, then everything can be removed
Append nothing to 'res' and return

3) Find the smallest character among first (n+1) characters
of 'str'.  Let the index of smallest character be minIndex.
Append 'str[minIndex]' to 'res' and recur for substring after
minIndex and for n = n-minIndex

buildLowestNumber(str[minIndex+1..len-1], n-minIndex).```

Below is the implementation of the above algorithm:

## C++

 `// C++ program for the above approach` `#include ``using` `namespace` `std;` `string removeKdigits(string num, ``int` `k)``{``    ``int` `n = num.size();``    ``stack<``char``> mystack;``    ``// Store the final string in stack``    ``for` `(``char` `c : num) {``        ``while` `(!mystack.empty() && k > 0``               ``&& mystack.top() > c) {``            ``mystack.pop();``            ``k -= 1;``        ``}` `        ``if` `(!mystack.empty() || c != ``'0'``){``            ``mystack.push(c);``        ``}``    ``}` `    ``// Now remove the largest values from the top of the``    ``// stack``    ``while` `(!mystack.empty() && k--)``        ``mystack.pop();``    ``if` `(mystack.empty())``        ``return` `"0"``;` `    ``// Now retrieve the number from stack into a string``    ``// (reusing num)``    ``while` `(!mystack.empty()) {``        ``num[n - 1] = mystack.top();``        ``mystack.pop();``        ``n -= 1;``    ``}``    ``return` `num.substr(n);``}` `int` `main()``{``    ``string str = ``"765028321"``;``    ``int` `k = 5;``    ``cout << removeKdigits(str, k);``    ``return` `0;``}`

## Java

 `// Java program for the above approach``import` `java.io.*;``import` `java.util.*;` `class` `GFG {` `    ``public` `static` `String removeKdigits(String num, ``int` `k)``    ``{``        ``StringBuilder result = ``new` `StringBuilder();``        ` `        ``// We have to delete all digits``        ``if` `(k >= num.length()) {``            ``return` `"0"``;``        ``}``        ``// Nothing to delete``        ``if` `(k == ``0``) {``            ``return` `num;``        ``}``        ``Stack s = ``new` `Stack();` `        ``for` `(``int` `i = ``0``; i < num.length(); i++) {``            ``char` `c = num.charAt(i);``          ` `            ``// Removing all digits in stack that are greater``            ``// than this digit(since they have higher``            ``// weightage)``            ``while` `(!s.isEmpty() && k > ``0` `&& s.peek() > c) {``                ``s.pop();``                ``k--;``            ``}``            ``// ignore pushing 0``            ``if` `(!s.isEmpty() || c != ``'0'``)``                ``s.push(c);``        ``}``      ` `        ``// If our k isnt 0 yet then we keep popping out the``        ``// stack until k becomes 0``        ``while` `(!s.isEmpty() && k > ``0``) {``            ``k--;``            ``s.pop();``        ``}``        ``if` `(s.isEmpty())``            ``return` `"0"``;``        ``while` `(!s.isEmpty()) {``            ``result.append(s.pop());``        ``}``        ``String str = result.reverse().toString();` `        ``return` `str;``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``String s = ``"765028321"``;``        ``int` `k = ``5``;``        ``System.out.println(removeKdigits(s, ``5``));``    ``}``}``// this code is contributed by gireeshgudaparthi`

## Python3

 `# Python program for the above approach``def` `removeKdigits(num, k):` `    ``n ``=` `len``(num)``    ``mystack ``=` `[]``    ` `    ``# Store the final string in stack``    ``for` `c ``in` `num:``        ``while` `(``len``(mystack) > ``0` `and` `k > ``0` `and` `ord``(mystack[``len``(mystack)``-``1``]) > ``ord``(c)):``            ``mystack.pop()``            ``k ``-``=` `1` `        ``if` `len``(mystack) > ``0` `or` `c !``=` `'0'` `:``            ``mystack.append(c)` `    ``# Now remove the largest values from the top of the``    ``# stack``    ``while` `len``(mystack) > ``0` `and` `k:``        ``mystack.pop()``        ``k ``-``=` `1``    ``if` `len``(mystack) ``=``=` `0``:``        ``return` `"0"` `    ``# Now retrieve the number from stack into a string``    ``# (reusing num)``    ``num ``=` `list``(num)``    ``while``(``len``(mystack) > ``0``):``        ``num[n``-``1``] ``=` `mystack[``len``(mystack)``-``1``]``        ``mystack.pop()``        ``n ``-``=` `1``    ``return` `"".join(num[n:])` `# driver code``str` `=` `"765028321"``k ``=` `5``print``(removeKdigits(``str``, k))` `# This code is contributed by shinjanpatra`

## C#

 `// C# program for the above approach``using` `System;``using` `System.Collections.Generic;``using` `System.Collections;``class` `HelloWorld {` `    ``static` `string` `removeKdigits(``string` `Num, ``int` `k)``    ``{``        ``char``[] num = Num.ToCharArray();``        ``int` `n = num.Length;``        ``Stack<``char``> mystack = ``new` `Stack<``char``>();` `        ``// Store the final string in stack``        ``for` `(``int` `i = 0; i < num.Length; i++) {``            ``while` `(mystack.Count > 0 && k > 0``                   ``&& mystack.Peek() > num[i]) {``                ``mystack.Pop();``                ``k = k - 1;``            ``}` `            ``if` `(mystack.Count > 0 || num[i] != ``'0'``) {``                ``mystack.Push(num[i]);``            ``}``        ``}` `        ``// Now remove the largest values from the top of the``        ``// stack``        ``while` `(mystack.Count > 0 && k > 0) {``            ``mystack.Pop();``            ``k = k - 1;``        ``}` `        ``if` `(mystack.Count == 0)``            ``return` `"0"``;` `        ``// Now retrieve the number from stack into a string``        ``// (reusing num)``        ``while` `(mystack.Count > 0) {``            ``char` `temp = mystack.Peek();``            ``num[n - 1] = temp;``            ``mystack.Pop();``            ``n = n - 1;``        ``}` `        ``return` `new` `string``(num).Substring(n);``    ``}` `    ``static` `void` `Main()``    ``{``        ``string` `str = ``"765028321"``;``        ``int` `k = 5;``        ``Console.WriteLine(removeKdigits(str, k));``    ``}``}` `// The code is contributed by Nidhi goel`

## Javascript

 ``

Output

`221`
• Time Complexity: O(N)
• Space Complexity: O(N)

Below is an optimized code in C++ contributed by Gaurav Mamgain

## C++14

 `// C++ program to build the smallest number by removing``// n digits from a given number``#include ``using` `namespace` `std;` `void` `insertInNonDecOrder(deque<``char``>& dq, ``char` `ch)``{` `    ``// If container is empty , insert the current digit``    ``if` `(dq.empty())``        ``dq.push_back(ch);` `    ``else` `{``        ``char` `temp = dq.back();` `        ``// Keep removing digits larger than current digit``        ``// from the back side of deque``        ``while` `(temp > ch && !dq.empty()) {``            ``dq.pop_back();``            ``if` `(!dq.empty())``                ``temp = dq.back();``        ``}` `        ``// Insert the current digit``        ``dq.push_back(ch);``    ``}``    ``return``;``}` `string buildLowestNumber(string str, ``int` `n)``{``    ``int` `len = str.length();` `    ``// Deleting n digits means we need to print k digits``    ``int` `k = len - n;` `    ``deque<``char``> dq;``    ``string res = ``""``;` `    ``// Leaving rightmost k-1 digits we need to choose``    ``// minimum digit from rest of the string and print it``    ``int` `i;``    ``for` `(i = 0; i <= len - k; i++)` `        ``// Insert new digit from the back side in``        ``// appropriate position and/ keep removing``        ``// digits larger than current digit``        ``insertInNonDecOrder(dq, str[i]);` `    ``// Now the minimum digit is at front of deque``    ``while` `(i < len) {` `        ``// keep the minimum digit in output string``        ``res += dq.front();` `        ``// remove minimum digit``        ``dq.pop_front();` `        ``// Again insert new digit from the back``        ``// side in appropriate position and keep``        ``// removing digits larger than current digit``        ``insertInNonDecOrder(dq, str[i]);``        ``i++;``    ``}` `    ``// Now only one element will be there in the deque``    ``res += dq.front();``    ``dq.pop_front();``    ``return` `res;``}` `string lowestNumber(string str, ``int` `n)``{``    ``string res = buildLowestNumber(str, n);``    ` `    ``// Remove all the leading zeroes``    ``string ans = ``""``;``    ``int` `flag = 0;``    ``for` `(``int` `i = 0; i < res.length(); i++) {``        ``if` `(res[i] != ``'0'` `|| flag == 1) {``            ``flag = 1;``            ``ans += res[i];``        ``}``    ``}` `    ``if` `(ans.length() == 0)``        ``return` `"0"``;``    ``else``        ``return` `ans;``}` `// Driver program to test above function``int` `main()``{``    ``string str = ``"765028321"``;``    ``int` `n = 5;``    ``cout <

## Java

 `// Java program to build the smallest number by removing``// n digits from a given number``import` `java.util.*;` `class` `GFG {``  ``static` `void` `insertInNonDecOrder(Deque dq,``                                  ``char` `ch)``  ``{` `    ``// If container is empty , insert the current digit``    ``if` `(dq.isEmpty())``      ``dq.addLast(ch);` `    ``else` `{``      ``char` `temp = dq.peekLast();` `      ``// Keep removing digits larger than current``      ``// digit from the back side of deque``      ``while` `(temp > ch && !dq.isEmpty()) {``        ``dq.pollLast();``        ``if` `(!dq.isEmpty())``          ``temp = dq.peekLast();``      ``}` `      ``// Insert the current digit``      ``dq.addLast(ch);``    ``}``    ``return``;``  ``}` `  ``static` `String buildLowestNumber(String str, ``int` `n)``  ``{``    ``int` `len = str.length();` `    ``// Deleting n digits means we need to print k digits``    ``int` `k = len - n;` `    ``Deque dq = ``new` `ArrayDeque<>();``    ``String res = ``""``;` `    ``// Leaving rightmost k-1 digits we need to choose``    ``// minimum digit from rest of the string and print``    ``// it``    ``int` `i;``    ``for` `(i = ``0``; i <= len - k; i++)` `      ``// Insert new digit from the back side in``      ``// appropriate position and/ keep removing``      ``// digits larger than current digit``      ``insertInNonDecOrder(dq, str.charAt(i));` `    ``// Now the minimum digit is at front of deque``    ``while` `(i < len) {` `      ``// keep the minimum digit in output string``      ``res += dq.peekFirst();` `      ``// remove minimum digit``      ``dq.pollFirst();` `      ``// Again insert new digit from the back``      ``// side in appropriate position and keep``      ``// removing digits larger than current digit``      ``insertInNonDecOrder(dq, str.charAt(i));``      ``i++;``    ``}` `    ``// Now only one element will be there in the deque``    ``res += dq.peekFirst();``    ``dq.pollFirst();``    ``return` `res;``  ``}` `  ``static` `String lowestNumber(String str, ``int` `n)``  ``{``    ``String res = buildLowestNumber(str, n);` `    ``// Remove all the leading zeroes``    ``String ans = ``""``;``    ``int` `flag = ``0``;``    ``for` `(``int` `i = ``0``; i < res.length(); i++) {``      ``if` `(res.charAt(i) != ``'0'` `|| flag == ``1``) {``        ``flag = ``1``;``        ``ans += res.charAt(i);``      ``}``    ``}` `    ``if` `(ans.length() == ``0``)``      ``return` `"0"``;``    ``else``      ``return` `ans;``  ``}` `  ``// Driver Code``  ``public` `static` `void` `main(String[] args)``  ``{``    ``String str = ``"765028321"``;``    ``int` `n = ``5``;``    ``System.out.println(lowestNumber(str, n));``  ``}``}` `// This code is contributed by Karandeep1234`

## Python3

 `# Python program to build the smallest number by removing``# n digits from a given number` `def` `insertInNonDecOrder(dq, ch):` `    ``# If container is empty , insert the current digit``    ``if` `len``(dq) ``=``=` `0``:``        ``dq.append(ch)` `    ``else``:``        ``temp ``=` `dq[``len``(dq) ``-` `1``]` `        ``# Keep removing digits larger than current digit``        ``# from the back side of deque``        ``while` `(temp > ch ``and` `len``(dq) > ``0``):``            ``dq.pop(``len``(dq)``-``1``)``            ``if` `(``len``(dq) > ``0``):``                ``temp ``=` `dq[``len``(dq) ``-` `1``]` `        ``# Insert the current digit``        ``dq.append(ch)``    ``return`  `def` `buildLowestNumber(``str``, n):` `    ``length ``=` `len``(``str``)` `    ``# Deleting n digits means we need to print k digits``    ``k ``=` `length ``-` `n``    ` `    ``dq ``=` `[]``    ``res ``=` `""` `    ``# Leaving rightmost k-1 digits we need to choose``    ``# minimum digit from rest of the string and print it``    ``i ``=` `0``    ``for` `i ``in` `range``(length``-``k ``+` `1``):``        ``# Insert new digit from the back side in``        ``# appropriate position and/ keep removing``        ``# digits larger than current digit``        ``insertInNonDecOrder(dq, ``str``[i])` `    ``i ``=` `i ``+` `1``    ``# Now the minimum digit is at front of deque``    ``while` `(i < length):``        ``# keep the minimum digit in output string``        ``res ``=` `res ``+` `dq[``0``]``        ` `        ``# remove minimum digit``        ``dq.pop(``0``)``        ` `        ``# Again insert new digit from the back``        ``# side in appropriate position and keep``        ``# removing digits larger than current digit``        ``insertInNonDecOrder(dq, ``str``[i])``        ``i ``=` `i ``+` `1` `    ``# Now only one element will be there in the deque``    ``res ``=` `res ``+` `dq[``0``]``    ``dq.pop(``0``)``    ``return` `res` `def` `lowestNumber(``str``, n):``    ` `    ``res ``=` `buildLowestNumber(``str``, n)``    ` `    ``# Remove all the leading zeroes``    ``ans ``=` `""``    ``flag ``=` `0``    ``for` `i ``in` `range``(``len``(res)):``        ``if` `res[i] !``=` `'0'` `or` `flag ``=``=` `1``:``            ``flag ``=` `1``            ``ans ``+``=` `res[i]` `    ``if` `len``(ans) ``=``=` `0``:``        ``return` `"0"``    ``else``:``        ``return` `ans` `# Driver program to test above function``str` `=` `"765028321"``n ``=` `5``print``(lowestNumber(``str``, n))` `# This code is contributed by Arushi Goel.`

## C#

 `// C# program to build the smallest number by removing``// n digits from a given number``using` `System;``using` `System.Collections.Generic;` `class` `GFG {``  ``static` `void` `insertInNonDecOrder(``ref` `LinkedList<``char``> dq,``                                  ``char` `ch)``  ``{``    ``// If container is empty , insert the current digit``    ``if` `(dq.Count == 0)``      ``dq.AddLast(ch);` `    ``else` `{``      ``char` `temp = dq.Last.Value;` `      ``// Keep removing digits larger than current``      ``// digit from the back side of deque``      ``while` `(temp > ch && dq.Count > 0) {``        ``dq.RemoveLast();``        ``if` `(dq.Count > 0)``          ``temp = dq.Last.Value;``      ``}` `      ``// Insert the current digit``      ``dq.AddLast(ch);``    ``}``    ``return``;``  ``}` `  ``static` `string` `buildLowestNumber(``string` `str, ``int` `n)``  ``{``    ``int` `len = str.Length;` `    ``// Deleting n digits means we need to print k digits``    ``int` `k = len - n;` `    ``LinkedList<``char``> dq = ``new` `LinkedList<``char``>();``    ``string` `res = ``""``;` `    ``// Leaving rightmost k-1 digits we need to choose``    ``// minimum digit from rest of the string and print``    ``// it``    ``int` `i;``    ``for` `(i = 0; i <= len - k; i++)` `      ``// Insert new digit from the back side in``      ``// appropriate position and/ keep removing``      ``// digits larger than current digit``      ``insertInNonDecOrder(``ref` `dq, str[i]);` `    ``// Now the minimum digit is at front of deque``    ``while` `(i < len) {` `      ``// keep the minimum digit in output string``      ``res += dq.First.Value;` `      ``// remove minimum digit``      ``dq.RemoveFirst();` `      ``// Again insert new digit from the back``      ``// side in appropriate position and keep``      ``// removing digits larger than current digit``      ``insertInNonDecOrder(``ref` `dq, str[i]);``      ``i++;``    ``}` `    ``// Now only one element will be there in the deque``    ``res += dq.First.Value;``    ``dq.RemoveFirst();``    ``return` `res;``  ``}` `  ``static` `string` `lowestNumber(``string` `str, ``int` `n)``  ``{``    ``string` `res = buildLowestNumber(str, n);` `    ``// Remove all the leading zeroes``    ``string` `ans = ``""``;``    ``int` `flag = 0;``    ``for` `(``int` `i = 0; i < res.Length; i++) {``      ``if` `(res[i] != ``'0'` `|| flag == 1) {``        ``flag = 1;``        ``ans += res[i];``      ``}``    ``}` `    ``if` `(ans.Length == 0)``      ``return` `"0"``;``    ``else``      ``return` `ans;``  ``}` `  ``// Driver program to test above function``  ``public` `static` `void` `Main()``  ``{``    ``string` `str = ``"765028321"``;``    ``int` `n = 5;``    ``Console.WriteLine(lowestNumber(str, n));``  ``}``}`

## Javascript

 `// javascript program to build the smallest number by removing``// n digits from a given number``function` `insertInNonDecOrder(dq, ch)``{` `    ``// If container is empty , insert the current digit``    ``if` `(dq.length == 0)``        ``dq.push(ch);` `    ``else` `{``        ``let temp = dq[dq.length - 1];` `        ``// Keep removing digits larger than current digit``        ``// from the back side of deque``        ``while` `(temp > ch && dq.length > 0) {``            ``dq.pop();``            ``if` `(dq.length > 0)``                ``temp = dq[dq.length - 1];``        ``}` `        ``// Insert the current digit``        ``dq.push(ch);``    ``}``    ``return``;``}` `function` `buildLowestNumber(str, n)``{``    ``let len = str.length;` `    ``// Deleting n digits means we need to print k digits``    ``let k = len - n;` `    ``let dq = [];``    ``let res = ``""``;` `    ``// Leaving rightmost k-1 digits we need to choose``    ``// minimum digit from rest of the string and print it``    ``let i = 0;``    ``for``(i = 0; i <= len - k; i++)` `        ``// Insert new digit from the back side in``        ``// appropriate position and/ keep removing``        ``// digits larger than current digit``        ``insertInNonDecOrder(dq, str[i]);` `    ``// Now the minimum digit is at front of deque``    ``while` `(i < len) {` `        ``// keep the minimum digit in output string``        ``res = res + dq[0];` `        ``// remove minimum digit``        ``dq.shift();` `        ``// Again insert new digit from the back``        ``// side in appropriate position and keep``        ``// removing digits larger than current digit``        ``insertInNonDecOrder(dq, str[i]);``        ``i = i + 1;``    ``}` `    ``// Now only one element will be there in the deque``    ``res = res + dq[0];``    ``dq.shift();``    ``return` `res;``}` `function` `lowestNumber(str, n)``{``    ``let res = buildLowestNumber(str, n);``    ` `    ``// Remove all the leading zeroes``    ``let ans = ``""``;``    ``let flag = 0;``    ``for` `(let i = 0; i < res.length; i++) {``        ``if` `(res[i] != ``'0'` `|| flag == 1) {``            ``flag = 1;``            ``ans += res[i];``        ``}``    ``}` `    ``if` `(ans.length == 0)``        ``return` `"0"``;``    ``else``        ``return` `ans;``}` `// Driver program to test above function``let str = ``"765028321"``;``let n = 5;``console.log(lowestNumber(str, n));` `// This code is contributed by Nidhi Goel.`

Output

`221`

Time Complexity: O(N)
Space Complexity: O(N)

Approach-2:

Let’s suppose the length of the given string num be n.so the result string will contain the length of n-k.

As we proceed to solve this problem we should make sure that the output string contains minimum values at their high weightage positions. so we ensure that by using a stack.

1. Return 0 if k >=n. and return num if k=0.
2. Create a stack and iterate through num string and push the value at that position if it is greater than the top element of the stack.
3. Iterate through the num string and if the integer value at that position is less than the top of the stack we will pop the stack and decrement k until we reach the condition where the top of the stack is less than the value we are looking at(while k>0) (by this we are making sure that most significant positions of the result are filled with minimum values).
4. If the k is still greater than 0 we will pop stack until k becomes 0.
5. Append the elements in the stack to the result string.
6. Delete leading zeroes from the result string.

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach` `#include ``using` `namespace` `std;` `string removeKdigits(string num, ``int` `k)``{``    ``int` `n = num.size();``    ``stack<``char``> mystack;``    ``// Store the final string in stack``    ``for` `(``char` `c : num) {``        ``while` `(!mystack.empty() && k > 0``               ``&& mystack.top() > c) {``            ``mystack.pop();``            ``k -= 1;``        ``}` `        ``if` `(!mystack.empty() || c != ``'0'``)``            ``mystack.push(c);``    ``}` `    ``// Now remove the largest values from the top of the``    ``// stack``    ``while` `(!mystack.empty() && k--)``        ``mystack.pop();``    ``if` `(mystack.empty())``        ``return` `"0"``;` `    ``// Now retrieve the number from stack into a string``    ``// (reusing num)``    ``while` `(!mystack.empty()) {``        ``num[n - 1] = mystack.top();``        ``mystack.pop();``        ``n -= 1;``    ``}``    ``return` `num.substr(n);``}` `int` `main()``{``    ``string str = ``"765028321"``;``    ``int` `k = 5;``    ``cout << removeKdigits(str, k);``    ``return` `0;``}`

## Java

 `// Java program for the above approach``import` `java.io.*;``import` `java.util.*;` `class` `GFG {` `    ``public` `static` `String removeKdigits(String num, ``int` `k)``    ``{``        ``StringBuilder result = ``new` `StringBuilder();``        ` `        ``// We have to delete all digits``        ``if` `(k >= num.length()) {``            ``return` `"0"``;``        ``}``        ``// Nothing to delete``        ``if` `(k == ``0``) {``            ``return` `num;``        ``}``        ``Stack s = ``new` `Stack();` `        ``for` `(``int` `i = ``0``; i < num.length(); i++) {``            ``char` `c = num.charAt(i);``          ` `            ``// Removing all digits in stack that are greater``            ``// than this digit(since they have higher``            ``// weightage)``            ``while` `(!s.isEmpty() && k > ``0` `&& s.peek() > c) {``                ``s.pop();``                ``k--;``            ``}``            ``// ignore pushing 0``            ``if` `(!s.isEmpty() || c != ``'0'``)``                ``s.push(c);``        ``}``      ` `        ``// If our k isnt 0 yet then we keep popping out the``        ``// stack until k becomes 0``        ``while` `(!s.isEmpty() && k > ``0``) {``            ``k--;``            ``s.pop();``        ``}``        ``if` `(s.isEmpty())``            ``return` `"0"``;``        ``while` `(!s.isEmpty()) {``            ``result.append(s.pop());``        ``}``        ``String str = result.reverse().toString();` `        ``return` `str;``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``String s = ``"765028321"``;``        ``int` `k = ``5``;``        ``System.out.println(removeKdigits(s, ``5``));``    ``}``}``// this code is contributed by gireeshgudaparthi`

## Python3

 `# Python program for the above approach``def` `removeKdigits(num, k):` `    ``n ``=` `len``(num)``    ``mystack ``=` `[]``    ` `    ``# Store the final string in stack``    ``for` `c ``in` `num:``        ``while` `(``len``(mystack) > ``0` `and` `k > ``0``               ``and` `mystack[``-``1``] > c):``            ``mystack.pop()``            ``k ``-``=` `1` `        ``if` `(``len``(mystack) > ``0` `or` `c !``=` `'0'``):``            ``mystack.append(c)` `    ``# Now remove the largest values from the top of the``    ``# stack``    ``while` `(``len``(mystack) > ``0` `and` `k):``        ``k ``-``=` `1``        ``mystack.pop()``    ``if` `(``len``(mystack) ``=``=` `0``):``        ``return` `"0"` `    ``# Now retrieve the number from stack into a string``    ``# (reusing num)``    ``num ``=` `list``(num)``    ``while` `(``len``(mystack) > ``0``):``        ``num[n``-``1``] ``=` `mystack[``-``1``]``        ``mystack.pop()``        ``n ``-``=` `1` `    ``return` `"".join(num[n:])` `# driver code``Str` `=` `"765028321"``k ``=` `5``print``(removeKdigits(``Str``, k))` `# This code is contributed by shinjanpatra`

## C#

 `// C# program for the above approach``using` `System;``using` `System.Collections.Generic;``class` `GFG {``    ``public` `static` `string` `removeKdigits(``string` `Num, ``int` `k){``        ``char``[] num = Num.ToCharArray();``        ``int` `n = num.Length;``        ``Stack<``char``> mystack = ``new` `Stack<``char``>();`` ` `        ``// Store the final string in stack``        ``for` `(``int` `i = 0; i < num.Length; i++) {``            ``while` `(mystack.Count > 0 && k > 0 && mystack.Peek() > num[i]) {``                ``mystack.Pop();``                ``k = k - 1;``            ``}``            ``if` `(mystack.Count > 0 || num[i] != ``'0'``)``                ``mystack.Push(num[i]);``        ``}`` ` `        ``// Now remove the largest values from the top of the``        ``// stack``        ``while` `(mystack.Count > 0 && k > 0) {``            ``mystack.Pop();``            ``k = k - 1;``        ``}`` ` `        ``if` `(mystack.Count == 0)``            ``return` `"0"``;`` ` `        ``// Now retrieve the number from stack into a string``        ``// (reusing num)``        ``while` `(mystack.Count > 0) {``            ``char` `temp = mystack.Peek();``            ``num[n - 1] = temp;``            ``mystack.Pop();``            ``n = n - 1;``        ``}``        ``return` `new` `string``(num).Substring(n);``    ``}`` ` `    ``public` `static` `void` `Main(){``        ``string` `str = ``"765028321"``;``        ``int` `k = 5;``        ``Console.WriteLine(removeKdigits(str, k));``    ``}``}``// THIS CODE IS CONTRIBUTED BY YASH AGARWAL(YASHAGARWAL2852002)`

## Javascript

 ``

Output

`221`

Time complexity: O(N)
Space complexity: O(N)