L14. Remove K Digits | Stack and Queue Playlist

[@brokegod5871]

While pushing the st.top() to the res, use res.push_back(st.top()) instead of res = res + st.top() for leetcode, otherwise you'll get a memory limit exceeded error. If I have to guess, it's because string append (+) has a O(string size) where a new string is created and then appended whereas push_back is only O(1)

[@KartikeyTT]

my code worked fine even after using res + st.top()
//here is the code
class Solution {
public:
string removeKdigits(string nums, int k) {
stack st;
for(int i=0; i

[@pranavmisra5870]

@@KartikeyTT it would work but using push-back gives better space complexity.

[@KartikeyTT]

@@pranavmisra5870 bro space complexity does not depend on that

[@glyoxal1933]

thanks

[@sukhii0220]

thanks brother

[@manishnaidu30]

Hey striver, learning so much from this series, thank you first of all, and another issue I wanted to tell is that there are no articles and linked youtube videos in the website, Hope they are done soon as you are doing this series so that it is helpful for people who haven't checked this playlist on youtube yet.

[@206-tusharkantimukherjee3]

This question is king of edge cases

[@pratikshah9219]

haha so true

[@Roshan__3006]

Great sir, I just started downloading and you just uploading these videos which Faster than BSNL

[@trashcan-md7cw]

Instead of using a stack of characters, It is better to use a String as a stack like we used a list as a stack in asteroid collision...
So we need not reverse the stack just remove the leading zeros and return it :) Happy Coding:)
CODE
string removeKdigits(string num, int k) {
string st = ""; // Use a string as the stack
int n = num.length();
// Traverse each digit in the string
for (int i = 0; i < n; ++i) {
// Pop digits from the stack if they are greater than the current digit
// and we still have digits to remove (k > 0)
while (!st.empty() && st.back() > num[i] && k > 0) {
st.pop_back();
k--;
}
// Push the current digit to the stack
st.push_back(num[i]);
}
// If k digits were not removed, remove from the end of the stack
while (k > 0) {
st.pop_back();
k--;
}
// Remove leading zeros
int start = 0;
while (start < st.size() && st[start] == '0') {
start++;
}
// Get the resulting number without leading zeros
string result = st.substr(start);
// If the result is empty, return "0"
return result.empty() ? "0" : result;
}

[@NetajiSaiSuru]

Exploring those Edge Cases and handling them is not everyone's cup of tea ❤‍🔥@Striver Kudoos!!

[@darkwarrior6767]

Instead of using stack to store answer you can directly use string and do operations on it using push_back and pop_back and back

[@ShahNawaz-cx3pi]

In some problems , brute force solution is more difficult that optimal solution in terms of implementation.
BTW------
Brute force solution: To remove k digits from the number, we can explore all possible combinations and choose the smallest number. This involves generating all subsequences of length n - k and selecting the smallest one
For Generating all the subsequences you can watch the striver's recursion playlist (There he has taught pick & non-pick method , after learning that method, recursion on arrays will become cakewalk).

[@venkatadhanunjayareddykana2819]

1:38

[@radhepatel6876]

Java Code: BTW This question is hard not medium
class Solution {
public String removeKdigits(String num, int k) {
Stack st = new Stack();
String result = "";
for(int i=0;i0 && (num.charAt(i)-'0')0){
st.pop();
k-=1;
}
if(st.isEmpty()){
return "0";
}
while(!st.isEmpty()){
result+=st.pop();
}
String res = "";
int index;
for(index=result.length()-1;index>0;index--){
if(result.charAt(index)!='0'){
break;
}
}
for(int i=index;i>=0;i--){
res+=result.charAt(i);
}
return res;
}
}

[@apmotivationakashparmar722]

Understood everything striver 😀😀

[@killerboy2387]

Hey,striver we wants Heap playlist.please....

[@shamanthhegde2820]

Hey Striver, I was able to solve this on my own thank you for that. to make it more simpler i removed the leading zero while inserting the numbers...
the logic is in case the stack is empty and i am going to insert a zero that doesn't make sense because that number is going to turn out to be leading zero that's all
public String removeKdigits(String num, int k) {
Stack st = new Stack();
int charPopped = 0;
String ans = "";
for(char ch:num.toCharArray()) {
while(!st.isEmpty() && st.peek() > ch && charPopped < k) {
st.pop();
charPopped++;
}
if(st.isEmpty() && ch == '0') continue;
st.push(ch);
}
while(!st.isEmpty() && charPopped < k) {
st.pop();
charPopped++;
}
if(st.isEmpty()) return "0";
int n = st.size();
for(int i=0; i

[@TOI-700]

JAI HIND veere | darna nhi h | ek call kar kabhi bhi available 24/7 for you veere

[@PawanKumar-hq6dy]

last reversal we can avoid if we use deque where we can add and remove from first and last

[@amitpandey8382]

I think time complexity will be O(4N)+O(K) .Inner while loop is also contributing o(N) time

[@saicharanchintha2327]

The inner while loop and the outer while loop will together take O(K) since only K elements can be removed from the stack overall.

[@oyeshxrme]

thanks bhaiya

[@saumay-z1]

So many edge cases.........Uffffff

[@shreyxnsh.14]

yeah, a lot of 'em

[@subee128]

Thanks

[@KartikeyTT]

tysm sir

[@tamoghnasaha2667]

The smallest number should be 1122 instead of 1219?

[@ayushaggarwal906]

you cannot change the order

[@manas4656]

here order should be maintained, we cannot change the position

[@DeadPoolx1712]

UNDERSTOOD;

[@yoddha621]

Mast maza agaya

[@Shivi32590]

understood

[@SibiRanganathL]

Understood

[@navinvenkat3404]

C++ sol =>
class Solution {
public:
string removeKdigits(string num, int k) {
stack st;
int n = num.size();
for(int i=0;i0 && (st.top() - '0') > (num[i] - '0')){
st.pop();
k--;
}
st.push(num[i]);
}
while(k>0 && !st.empty()){
st.pop();
k--;
}
string result = "";
while(!st.empty()){
result += st.top();
st.pop();
}
reverse(result.begin() , result.end());
int start = 0;
while(start < result.size() && result[start] == '0'){
start++;
}
result = result.substr(start);
return result.empty() ? "0" : result;
}
};

[@shreyxnsh.14]

Thanks, was able to do this by myself, here is the C++ code:
class Solution {
public:
string removeKdigits(string num, int k) {
stack st;
for(const char& c: num){
while(!st.empty() && c0){
st.pop();
k--;
}
st.push(c);
}
while(k>0){
st.pop();
k--;
}

string res = "";
while(!st.empty()){
res.push_back(st.top());
st.pop();
}
reverse(res.begin(), res.end());

int i = 0;
while(res[i] == '0'){
i++;
}
res = res.substr(i);
if(res == "")
return "0";
return res;
}
};

[@BeAcoder1011]

[@charuprabha8714]

Hi but how to think that we have to use stack to solve this problem🙂

[@shreyxnsh.14]

you dont need to, you can do this by creating a new string and operating on it

[@arunraj4383]

[@irfanmohammad2132]

Memory Limit Exceeded

[@KalingaAbhisek]

Java Code
class Solution {
public String removeKdigits(String num, int k) {
Stack st = new Stack();
for(int i=0;inum.charAt(i)-'0' && k>0){
st.pop();
k--;
}
st.push(num.charAt(i)-'0');
}
while(k>0 && !st.isEmpty()) {
st.pop();
k--;
}
String res="";
while(!st.isEmpty()){
res+=st.pop();
}
StringBuilder sb = new StringBuilder(res).reverse();
while(sb.length()>0 && sb.charAt(0)=='0'){
sb.deleteCharAt(0);
}
return sb.length()>0?sb.toString():"0";
}
}

[@Messi23485]

Can anyone please explain why he minus 0 in while condition during comparison of stack top with current character of string

[@shashank_0807]

To convert char to int.

[@valendradangi1822]

It will work even if you don't do that

[@shreyxnsh.14]

Solution without stack:
class Solution {
public:
string removeKdigits(string num, int k) {
string newstr = "";
for(const char& c: num){
while(!newstr.empty() && c0){
newstr.pop_back();
k--;
}
newstr.push_back(c);
}
while(k>0){
newstr.pop_back();
k--;
}

int i = 0;
while(newstr[i] == '0'){
i++;
}
newstr = newstr.substr(i);
if(newstr == "")
return "0";
return newstr;
}
};

[@tarunyadav6617]

Heyy are u from Dtu or which college ?

[@MJBZG]

your new lectures are becoming difficult to understand

[@satyen4659]

but topics are also complex

[@shreyxnsh.14]

this should have been easy if you've been following his playlist

[@sanatgupta1562]

Nobody asked for your opinion ​@@shreyxnsh.14

[@sanatgupta1562]

Nobody asked for ur opinion ​@@shreyxnsh.14

[@valendradangi1822]

#include
using namespace std;
class Solution
{
public:
string removeKdigits(string num, int k)
{
// char imp
stack st;
int n = num.size();
int removed = 0;
for (int i = 0; i < n; i++)
{
while (!st.empty() && removed < k && st.top() > num[i]) // Don't do >= Dry Run 1,2,2,2,2,5
// k > 0 && st.top() - '0' > num[i] > '0'
{
removed++;
st.pop();
}
st.push(num[i]);
}
while (removed < k)
{
st.pop();
removed++;
}
if(st.empty())return "0"; // Makes code faster anyways
// second last if can handle it
int idx = st.size();
string ans(idx, '%'); // Prevents reversing the string
while (!st.empty())
{
ans[--idx] = st.top();
st.pop();
}
int i = 0;
while (i < ans.size() && ans[i] == '0')
i++;
// The erase function removes i characters
// from given index (0 here)
ans.erase(0, i);
// If are using reversal
// while(ans.size() > 0 && ans.back() == '0')
// ans.pop_back();
if (ans.empty())
return "0";
return ans;
}
};
// This is a good question to study edge cases.
// 1) k char may not be removed
// 2) ans may contain leading zeroes
// 3)
// TC => O(N) + O(K) + O(N) + O(N)
// SC => O(N) + O(N)
int main(){
return 0;
}

[@KartikeyTT]

tysm sir

[@rutujashelke4208]

Understood

[@aryankumar3018]

understood

[@abhinavabhi3568]

Understood