Dp 25. Longest Common Subsequence | Top Down | Bottom-Up | Space Optimised | DP on Strings
ฝัง
- เผยแพร่เมื่อ 28 ก.พ. 2022
- Lecture Notes/C++/Java Codes: takeuforward.org/dynamic-prog...
Problem Link: bit.ly/3pvkqUd
Pre-req for this Series: • Re 1. Introduction to ...
a
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Full Playlist: • Striver's Dynamic Prog...
In this video, we solve the LCS Dp, this is the first problem on the pattern Strings on DP.
If you have not yet checked our SDE sheet, you should definitely do it: takeuforward.org/interviews/s...
You can also get in touch with me at my social handles: linktr.ee/takeUforward
46:05 for space optimization we don't require the loop for prev as the values are already zero in it.
If someone is following Striver's series then this LCS is also a cakewalk..#Striver gives u confidence and you are no longer scared of dp😁
Understood and you've completely changed my life. Doesn't matter if I get placed in a good company or not but the quality of this lectures are supreme and I will carry this knowledge for rest of my life.
Keep trying. It will work out. Dream for the day when you are so good that you REJECT your dream company.
@@AdityaKumar-be7hx Requires god level consistency to do that.
You will probably forget the concepts, since these are never used in the industry. What a satire !
🤣🤣@@idris110
brother are you placed
In the future, Mr. Vikramaditya will go down as the G.O.A.T online DSA teacher.
Yes he is GOAT
I am the future
He is already for me.
putting your years of hardwork in some videos ,how lucky we are,Thanks a lot striver bhaiya for everything you are doing for us
Protect this guy at all costs. Thank you sir for all your hard work in making this video.
Everyone should be protected bro
15:34 "You kow i am hearing you" in the background
So?
This was life-changing. Thank you Striver, you taught what paid courses could not for so many years. I wish I had discovered your site earlier; it would have saved so much time and energy.
understood bhaiya!!! after a very long time i am back to your channel....previously i was doing a race that as soon as you upload the video, i should watch it then n there, before the next video gets uploaded in this dp series, but due to some busy schedule, i lost the race. yeah, but your quality and energy is still the same as your starting videos
I used to be scared of Dp when I started coding journey, but when I am actually doing it, it's cake walk. Thankyou striver for sharing your knowledge.
I am very grateful to you for uploading this playlist. This is great!!!!!!!!!!!!!!!!!!!!!
Understood.
No other video on the topic will offer you a better explanation than this! This is just pure teaching excellence! Subscribed.
You're a great teacher. If possible, please do problems about DP on tree. I struggle with them. Thank you!
bro!!!!!! What an explanation, absolutely brilliant. I am starting to love coding now. Thanks
striver help this brother out!
how you are staying disicplined?
so far you are single ,right?
how you are avoiding every distractions and focusing on your goal
please make a video on this brother!!
Its taking while to digest this information for me ,
Just imagine the efforts this guy is adding to make it available for everyone.
absolutely minds bogling.i am not kidding i wasn't able to solve simple recusion questions few weeks ago now i can solve medium and hard dp questions on my own without watching videos.all i can say thank you striver.
What an easy explanation, loved it !
You can't find better explanation than this, Brilliant!!
bro striver you have taught so well that i didnt even need to watch the video,i solved the problem in all three ways,thaank you very much❤
understood, always in awe of you genius and hard working you are:)
UNDERSTOOD.....Thank You So Much for this wonderful video............🙏🏻🙏🏻🙏🏻🙏🏻🙏🏻🙏🏻
Nice explanation raj bhaiyan or dude, I might call you angel!🥰🥰
simple space optimization technique : just 2,3 chanes
instead of using 2 different vectors we can just change dimension of dp to [no of previous rows we wanred + 1][no of colum]
int longestCommonSubsequence(string s1, string s2) {
vectordp(2, vector(s2.size()+1, 0)); // changing dimension as per need
for(int i=1; i
Giving my best shot for preparing for my placements. Let's see what happens in upcoming months. Great Content
I solved with different tabulation approach (without shifting index) as below:
int m=text1.length();
int n=text2.length();
int[][] dp = new int[m][n];
int temp=0;
for(int i=0; i
same
US ❤
I should mention I am solving DSA consistently from past 1 year. Encountered this question a lot of times.
But till date this is the best explanation boss❤
If someone is using java to write the code, in space optimization approach there will be error in a testcase, to resolve that error replace prev = curr with prev = curr.clone(); or else use the code snippet
int [] temp = prev;
prev = (curr);
curr = temp;
Because in java prev = curr will not create a new array but it will point both the references to same address.
Your program would work fine.
understood.wonderful. amazing. hats-off. unmatchable.
I’m just so thankful to you for this wonderful series, I’m honestly enjoying it dude, surely you’re one of the best!
I even did the problem by myself so easily, i was shocked believing😂
me too😂
Understood! Awesome explanation as always, thank you very much!!!
Can we do using single array if we denote prev[j-1] in a variable, keep updating it after every iteration of inner loop ?
Understood,you are doing the great job , very thankful to you
I was able to see that curr= prev; mistake as you were typing it. thanks to you Striver. you are the best
Thank you so much bhaiya, I always heard of this question as one of the most difficult ones. But by following your DP series, I did it on my own. You can easily sell this quality content for ₹ 50k but you're giving it for free. BEST TEACHER EVER ♥♥♥♥
You are amazing ❤️.
Thanks a lot for this valuable content 🙌🙇🙇
In Tabulation we are declaring vector with values zero. So we can skip first two loops.
UNDERSTOOD...
Thanks striver for the video... :)
Why can't we put the base case at index 0? Especially for LCS - DP on strings related questions?
In interviews do we need to show the overlapping subproblems by taking longer examples or we can simply move on with memoization?
take till 5 to 6 steps., if recruiter does want , want he will tell u., to move to next step.
Thanks bro the Playlist is top notch...am getting a lot better...Thanks again
awesome, understood..was pretty easy due to what you've alrready taught
Very nice explanation sir, Thank you!
hey great video but isn't the auxiliary stack space supposed to be max(s.size(), t.size())?
Striver, why can't we return INT_MIN , in the base case. Because if we return 0, it will be added to 1 and the resultant number will be considered for maximum. And even if it is not the maximum, it will be returned. Your explanation is very intuitive.
Loved the Explanation!!Understood
what an explanation thanks for your time and such video
understood,previously I mugged up the tabulation formula for the exam ,but now I know how it came.
Understood, Thank you so so much.
almost all of the questions in this playlist can be space optimized to not just O(2m) but O(m) if you apply some logic and visualize how the table is being filled.
hey man i know you are a very good teacher , but you are also a very good human being.
Understood. Only one point, if you are doing the tabulation to eliminate the O(m*n) S.C and then including (m+n) extra space for an extra row and column, it seems a bit redundant.
m+n
hey striver this is regarding ur tuf website......one feedback....can you add another checklist to mark important or tough nut questions so that for revision we can se it carefully again....because this subsequence is screewing up my brain :)
We don't need to add base case while bottom-up in case of index shifting. If we just replace "-1" with "0" while initializing dp vector. Isn't it?
I deeply understand recursion, memoization, tabulation and space optimization.
Man the content is gold.
Just one word , Beautiful!
Thank you Striver for such a wonderful explanation. Finally understood it intuitively. 💯💯
Can also be done using single array if we preserve cur[j-1] in a prv variable
Yeah.. you guys are learning fast 😍
@@takeUforward thanks to your playlist💯
@@tejasghone5118 can u provide the code
@@jaykumargupta7307
int longestCommonSubsequence(string s1, string s2) {
int n = s1.length(), m = s2.length();
vector cur(m+1, 0);
int prev;
for(int ind1=1; ind1
can u give the video link in which he taught this space opt technique
seriously you are a legit!
understood ,thanks🙌
Again @takeUforwad, just letting you know, again that's stuff you have done, is GOD's own work, thank you for from bottom of my heart, thanks !!!
Thanks for great explanation striver
Very good explanation. Understood. Thanks.
Dhonnobad Brother
You are great
Thank You Very much Striver !
IN TABULATION WE CAN STORE 0 INTIALLY AND skip the base case loops which is used to store 0 in 0th row and column and same for space optimization
Decode ways is a new kind of pattern on dp on strings i think and that can be covered
Understood, sir. Thank you very much.
Life Lesson: If Raj tells you to go revise a video, then go revise that video. It's really hard not to understand something once you watch everything in the order he presents.
Was able to solve this on Leetcode by myself. Thanks Striver !!!
public int longestCommonSubsequence(String s1, String s2) {
int n1 = s1.length(),n2 = s2.length();
int[] prev = new int[n2];
//base cases
if(s1.charAt(0)==s2.charAt(0)) prev[0]=1;
for(int i=1;i
Awesome lecture. Understood!
Thanks a lot striver
Can anyone please explain why (index1 == 0 || index2 == 0) has not been considered as base case here as we did in DP of arrays?
In DP of Arrays, I was trying to use (index < 0) as the base case, but when I saw Striver's video, I changed my approach to his, where he was considering (index == 0 ) as the base case. But now, in this, he is using the other way around.
Can someone please explain why?
You can use index1 == 0 || index2 == 0 but then it will have 2 cases 1) if(index1 == 0 && index2 == 0) return s1[index1] == s2[index2] ;
2) if(index1 == 0 || index2 == 0) \\ using loop find the single character(having index == 0) in other string if present return 1 else return 0
therefore it is difficult to find and index < 0 base case is much easier so it's better to write that
Thanks Striver. Understood LCS.
Without right shifting the Indexes, still we could do it as default initialise with 0 and then start from i=0 and j=0 but adding constraints of out of bounds.
#include
//Tabulation Solution
int lcs(string s, string t)
{
int n1=s.size();
int n2=t.size();
vectordp(n1,vector(n2,0));
for(int i=0;i=0){
dp[i][j]=1+dp[i-1][j-1];
}
else{
dp[i][j]=1;
}
}
else{
int a=0,b=0;
if(i-1>=0){
a=dp[i-1][j];
}
if(j-1>=0){
b=dp[i][j-1];
}
dp[i][j]=max(a,b);
}
}
}
return dp[n1-1][n2-1];
//Write your code here
}
I did the same with java code
he is the example of beauty with brains
Does interviewers expect us to know the tabulation directly? because I can solve tabulation only after analysing recursion
No, Its not expected to directly jump to the tabulation. In Contrast, the interviewer will rather be impressed, that you moved your solution trom recursion to memoised to space optimised solution
@@viasmit Exactly, they want to know how we reach the tabulation from the recursion
@@parthsalat is space optimization after tabulation essential?
@@divyanshsharma8401 An average interview question lasts around 45 mins. If you have time remaining, then yes, go for space optimisation.
But it isn't compulsory.
Understood ❤
understood bruh, plz make video on sliding window technique too.
Is it possible to represent every dp problem by recursion tree?
Understood boss man!
who agrees that this dp series is one of the best on TH-cam
Not one of the best no. THE BESTTTTT!!!!!
did it in 1st attempt without seeing the video, shifting index to right was new to me
Understood Sir!
GOAT for a reason❤❤
Today, I solved my first ever dp problem in a contest just by watching this video...
Codechef Starter 71 Longest Common Subsequence problem
very well explained step by step
In case of Java while doing the space optimization when we reassign prev = current; why do we require to clone it? It was working when we were simply assigning the last time.
Edit: I think I was creating current array inside the first for loop and thus it didn't require cloning.
Length of both the String can be different, how the o(N) space solution will work for this?
In this qye shifting humne khali comparsion wale usme kyon kia hain like wile function call humko I ko i-1 se replace nahi karna?
UNDERSTOOD!
in tabulation method to avoid getting negative values this condition can be used
if(i1==0 || i2==0){
if(s1.charAt(i1)==s2.charAt(i2)){
dp[i1][i2]=1;}
else if(i1==0 && i2!=0){
dp[i1][i2]=dp[i1][i2-1];}
else if(i1!=0 && i2==0){
dp[i1][i2]=dp[i1-1][i2];
}
}
Understood sir ! 🙏🙏
very good explanation
i myself did it 1array space optimisation ! quite impressive how i learnt that fast 😀😀
can you please share the code of 1 array optimisation , my 1 array code giving me wrong answer on leetcode testcase 23 , i saw a solution where someone uses 2 extra variables to do in 1 array. plz reply
how we computed the time complexity of recursive solution?
I was always scared of these LCS and all but not anymore thanks to striver
Bhaiya ,why cant we solve this question like if we have got two strings then we are eliminating the characters from both strings that are uncommon and dont have frequency 1 (checking that in common characters)..ultimately we will get two resultant strings and if they are equal that is longest subsequence and if not there is no longest common subsequence present ..pls clear my doubt
Understood !!
Love the concept ✨