L7. Number of Substrings Containing All Three Characters | 2 Pointers and Sliding Window Playlist

Notes/Codes/Problem links under step 10 of A2Z DSA Course: takeuforward.org/strivers-a2z...
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[@artofwrick]

These questions are very important for contests . The first question is always a string question where you have to generate subarray. For arrays, questions from PREFIX sum comes often

[@namannema3349]

i hope striver one day i will build logic like you

[@mayank_singh_43]

This problem of counting substring and subarrays always confused me in any contest , now I learned the concept of how to do this. THanks a lot striver bhaiya

[@sauravkumar-ln7zh]

i did this question based on the logic based on previous question → We need to monitor every character in the sliding window. → For this, we use a map to keep track of the number of each character present in the sliding window. → If the number of distinct characters exceeds k, we start removing characters from the back until the size of the map is less than or equal to k. → If the count of a certain character becomes zero while removing it from the back, we must erase it from the map to decrease the map's size. class Solution { public: int plzhelp(string s, int k) { int i = 0; int j = 0; unordered_map mp; int count = 0; while (j < s.length()) { mp[s[j]]++; while (mp.size() > k) { mp[s[i]]--; if (mp[s[i]] == 0) { mp.erase(s[i]); } i++; } count += (j - i + 1); j++; } return count; } int numberOfSubstrings(string s) { int k = 3; int count = plzhelp(s, k) - plzhelp(s, k - 1); return count; } };

[@sauravdhar1696]

can you explain why you did => int count = plzhelp(s, k) - plzhelp(s, k - 1) ??

[@Rahul_Mongia]

@@sauravdhar1696 kyo ki plzhelp(s,k) will return all substring having characters

[@bhargav.v.m.193]

@@Rahul_Mongia can this be avoided by making count += (j - i + 1); j++; to if(mpp(mp.size() == k){ count += (j - i + 1); } j++;

[@user-kx1sv7lq4k]

I was not able to understand this question's approach but you have done it :)

[@2amCoder]

the reason you were good at cp. i tried many things with this solution but none of them were close good as this last explanation

[@ManishKumar-dk8hl]

optimal :- class Solution { public int numberOfSubstrings(String s) { int len=0; int r=0; int[] arr=new int[3]; Arrays.fill(arr,-1); while(r

[@ratishjain2718]

insane approach

[@cheezy_cheez]

best explanation! especially the part where you mentioned why even omiting the if checking would be ok, I was bamboozled haha

[@Adarsh-fg5gs]

i did somewhat diffrent as TOtal no of subarrays - subarrays with at most 2 distinct characters and it becomes same as previous question int countSubstring(string s) { //Total no of subarrays with n characters =n(n+1)/2 int n=s.size(); int total=n*(n+1)/2; //now write code to find for at most 2 distinct characters int acnt=0,bcnt=0,ccnt=0,res=0,l=0,r=0; while(r0 && bcnt>0 && ccnt>0){ if(s[l]=='a')acnt--; if(s[l]=='b')bcnt--; if(s[l]=='c')ccnt--; l++; } res+=(r-l+1); r++; } return total-res; //total no of subarrays-subarrys with at most two distinct }

[@rlm3227]

The optimised solution is a very clever sol

[@sunitsable2752]

amazing logic!!!

[@ganeshjaggineni4097]

NICE SUPER EXCELLENT MOTIVATED

[@moonlight-td8ed]

mind blown dude... crazy optimal soln

[@Arya20012]

what a grate solution,just love this,thank u brother

[@abhinayrk985]

Outstanding

[@sobhansahoosubh3873]

we also do like this way int numberOfSubstrings(string s) { int n = s.size(); int l = 0,r = 0,count = 0; unordered_map mp; while(r < n) { mp[s[r]]++; while(mp['a'] >= 1 && mp['b'] >= 1 && mp['c'] >= 1 ) { count = count + (n - r); mp[s[l]]--; l++; } r++; } return count; }

[@codeman3828]

Understood. great

[@subee128]

Thanks

[@KratiGarg-ue1ph]

I tried the approach you gave in the binary subarray question (number of total subarrays(n*(n+1)/2) - the subarrays where mapcount < 3. that also worked. Please give tips on how to approach a question like this!

[@tanishqtyagi1465]

The series is dam good!! 🤍🤍💯

[@rajalakshmis7308]

Thank you, Striver. Before watching this video, I just solved it using your previous lecture pattern. But, the approach you used is the best among all. import java.util.HashMap; public class Solution { public static int countSubstring(String s){ // Write your code here. // to find the tot subarray count int res = s.length()*(s.length()+1)/2; // return tot subarray - subarray which has atmost any 2 characters from a,b,c. return res-solve(s); } // function to find a subarray which has atmost any 2 character from a,b,c public static int solve(String s){ HashMap map = new HashMap(); int left=0,count=0; for(int right=0;right2){ map.put(s.charAt(left), map.get(s.charAt(left))-1); if(map.get(s.charAt(left))==0) map.remove(s.charAt(left)); left++; } count+=right-left+1; } return count; } }

[@surendharv795]

Your Solution is good , but it is failing in the 48 th test case in leetcode.

[@techyguyaditya]

This took a bit of time to understand for optimal approach. I was literally trying to derive mathematical formula which only passes the test case shown in video, further optimizing the code. But the edge case is that, L may not update in other test cases. Basic approach: Find left value of minimum sliding window in each iteration (start finding once a,b,c gets it's value other than -1). Then basically, for each iter, ctr += 1 + L (where L is leftmost index of window, min(a,b,c)). Striver said to omit if statement, because 1 + (-1) = 0. I disagree with that, because if you see low level programming, the unnecessary write operation happens to the memory even if the value remains the same. Write operations are generally considered as costly operation. Even if it's for 1 extra line of code, it will prevent the costly write operation just by having read operation, further optimizing the code.

[@akay5797]

Mindblown

[@saurabhchaudhari4157]

//Another Optimized Approach class Solution { public: int numberOfSubstrings(string s) { int n=s.length(); int cnt=0; int left=0; int right=0; unordered_mapmpp; while(right

[@aamna5243]

bhai farishta h tu.

[@ayanahmad12]

Literally 🔥✅

[@PawanKumar-tj2xk]

@@ayanahmad12 simp

[@ujjwalkashyap9196]

Please update the site also with all the upcoming videos 🙏

[@shivisingh9975]

Understood!

[@square-kstudios9561]

Great explanation! How does one come up with a solution like this in the constraints of an interview though, if we haven't seen it ever in the past? Some companies only give you 15 mins to come up with a solution, explain it, dry run it, code it and then provide the time/space complexity.

[@ajithshetty1684]

did you get the answer to your question?

[@dipendrasingh4874]

thank you bhaiya ... please tcs nqt segment i was solving them but u removed that from site .....(or i am not able to find it )please add thank you

[@shashankvashishtha4454]

understood

[@RonitTejani]

Legit God

[@rushidesai2836]

This looks more like CP question rather than an interview question.

[@adityapandey23]

Understood

[@AkOp-bf9vm]

the line if(hash[0]!=-1 && ...) is not compulsory, the code will work fine without this line because -1+1=0 and if 0 is added to Count it didn't impact the answer

[@Flash-qr5oh]

WoW!

[@rider5584]

here is the java code = class Solution { public int numberOfSubstrings(String s) { // Array to store the latest positions of characters 'a', 'b', and 'c' int[] latestPosition = new int[] {-1, -1, -1}; // This will hold the count of valid substrings int answer = 0; // Iterate over each character in the string for (int i = 0; i < s.length(); ++i) { char currentChar = s.charAt(i); // Update the latest position of the current character latestPosition[currentChar - 'a'] = i; // Find the smallest index among the latest positions of 'a', 'b', and 'c' // and add 1 to get the count of valid substrings ending with the current character int minPosition = Math.min(latestPosition[0], Math.min(latestPosition[1], latestPosition[2])); answer += minPosition + 1; } return answer; // Return the total count of valid substrings } }

[@socify4410]

fabolous

[@saakshishekhar237]

int numberOfSubstrings(string s) { vector lastSeen(3,-1); int cnt = 0; for(int i=0; i

[@sakethsenapathi8323]

i didnt get what is lastseen[s[i] - 'a'] = i part can u please explain.

[@ManishKumar-dk8hl]

@@sakethsenapathi8323 s[i] character represent kr raha h aur jb unme 'a' minus hoga too a-a=0 ; b-a=1 ;c-a =2 ayega then lastseen wali array k index pr string k character ka index store hoyega . index 0 of array=a, index 2= b,index 2=c

[@animexworld6614]

My question is what is your favorite colour

[@angeldeveloper]

❤👍

[@adilkevin6220]

Document link is not attached for some of the program

[@expanse8846]

Another solution using previous video method class Solution { public: int numberOfSubstrings(string s) { vectorhashh(3,0); int l=0,r=0,n=s.size(); int ct=0; // vectordis; while(r0 && hashh[1]>0 && hashh[2]>0) { ct+= n-r; hashh[s[l]-'a']--; // dis.push_back(ct); l++; } r++; } // for(int i=0;i

[@nirbhaysingh7971]

#include int countSubstring(string s){ // Write your code here. int n = s.size(); int left = 0; int right = 0; map mpp; int cnt = 0; while(right

[@omkarsawant9267]

#include #include #include #include using namespace std; // Function to count the number of substrings containing all three characters 'a', 'b', and 'c' pair numberOfSubstrings(string s) { // Hashmap to store the count of 'a', 'b', and 'c' in the current window unordered_map count; int left = 0, result = 0; vector substrings; // Iterate over the string with 'right' as the end of the window for (int right = 0; right < s.length(); ++right) { // Increment the count of the current character count[s[right]]++; // Check if all three characters are present in the current window while (count['a'] > 0 && count['b'] > 0 && count['c'] > 0) { // If yes, add all possible substrings starting from the current 'left' to 'right' result += s.length() - right; // Capture the substrings for (int k = right; k < s.length(); ++k) { substrings.push_back(s.substr(left, k - left + 1)); } // Move the left end of the window to the right count[s[left]]--; left++; } } return {result, substrings}; } int main() { string s = "abcabc"; auto result = numberOfSubstrings(s); cout

[@CE_113_Katyayni]

sir your brute force approach is actually wrong because when we sum the hash[0]+hash[1]+hash[2] ==3 here it may be the case that hash[1]=0 and hash[0]=2 in this case also the if state would be true and cnt will increase which is actually wrong

[@rohitvishwakarma7046]

Yeah, I think he meant to take the size of hashmap , hope you get it.

[@azizkavas6993]

No such case is possible because hash doesnt count the number of occurance instead it just sign the related index. If you try yourself with sample code you will see what I mean. Kind regards.

[@ashnidahiya8347]

This is a brute force approach using sets, hope it helps: int countSubStrings(string str, int k) { set st; int count = 0; int n = str.length(); for(int i=0;i

[@sparksfly4421]

his code in brute force is correct. The hash array only gets assigned a truthy value (1) at the index of the given alphabet (where index(a) = 0, index(b)=1, index(c)=2) if it happens on the given letter in the string. It can never increment beyond that since it's an assignment operator

[@doremon81072]

demm man

[@user-fw4kz3bb4g]

EASIEST APPROACH (C++) class Solution { public: int numberOfSubstrings(string s) { int left=0, right=0,count=0; vectorarr(3,0); while(right0 && arr[1]>0 && arr[2]>0){ count+=s.size()-right; arr[s[left]-'a']--; left++; } right++; } return count; } };

[@Bhai9866]

public int numberOfSubstrings(String s) { int[] lastSeen = new int[3]; Arrays.fill(lastSeen, -1); int cnt = 0; for (int i = 0; i < s.length(); i++) { char c = s.charAt(i); lastSeen[c - 'a'] = i; if (lastSeen[0] != -1 && lastSeen[1] != -1 && lastSeen[2] != -1) { cnt += (1 + Math.min(lastSeen[0], Math.min(lastSeen[1], lastSeen[2]))); } } return cnt; }

[@gocrazy7362]

unordered_map mp; int l = 0,r=0,cnt = 0; while(r=1 && mp['b']>=1 && mp['c']>=1){ cnt += s.size() - r; mp[s[l]]--; l++; } r++; } return cnt;

[@KartikeyTT]

If someone can debug this solution then they are real genius class Solution { public: int numberOfSubstrings(string s) { map map; int l = 0; int r = 0; int count = 0; int minValue=0; while (map.size() != 3) { map[s[r]] = r; r++; } while (r < s.length()) { minValue=INT_MAX; for (const auto& pair : map) { cout

[@ShubhamMIshra-hv5nz]

CODE FOR THEOPTIMISED SOLUTION!!!!! class Solution { public: int numberOfSubstrings(string s) { ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); int count =0; int length=s.size(); int lastSceen[3]={-1,-1,-1}; for(int i =0; i < length; i++){ lastSceen[s[i]-'a']=i; if(lastSceen[0] != -1 && lastSceen[1]!= -1&& lastSceen[2]!=-1) { count += min(lastSceen[0],min(lastSceen[1],lastSceen[2])) +1; } } return count; } }; whats up??

[@user-zm6nk5mw9i]

i wrote a O(n) code but it gives only 36% better i dont know why and how can i improve it further? class Solution { public: int numberOfSubstrings(string s) { int l=0; int n=s.size(); unordered_map index; int ans=0; for(int r=0;r

[@anmolvanced3262]

viewed multiple times .. but your explanation is not good .. sorry to say!

[@KartikeyTT]

If you want to find minimum of three elements in cpp. You can do it like this- int temp = min(arr[0],arr[1]); int lowestI = min(temp, arr[2]);

[@AnujGupta-xi5ep]

For one liner you can do : int ans = min({arr[0], arr[1], arr[2]});

[@KartikeyTT]

@@AnujGupta-xi5ep ohhh, i was doin without the braces and was getting error, so i thought it wasn’t possible in cpp

[@adityababu3405]

int numberOfSubstrings(string s) { vector lastSeen(3,-1); int cnt = 0; for(int i=0; i