Stacks : Interview Questions | Leetcode Problems | Lecture 55 | Java and DSA Course

In this video, Raghav Sir will teach you TOP INTERVIEW questions of Stacks. This is Lecture 55 of the JAVA & DSA series and the second video of Stack. Various questions and patterns of questions are covered in this lecture. Infix, Prefix, Postfix evaluations and conversions and related questions of Stacks will be covered in the next Lecture in detail, coming VERY SOON.

Annotated Notes Link:

00:00 Introduction
02:26 Q1 : Balanced Brackets
23:57 HW 2 : Minimum No. of brackets removed to make sequence balanced
26:01 HW 3 : Valid bracket sequence
29:54 Q4 : Remove consecutive subsequences
56:52 Q5 : Next Greater Element
1:30:42 HW 6 : Stock span problem
1:47:12 Q7 : Next Greater Element - Method 2
1:55:04 Q8 : Largest area Histogram
2:39:02 Q9 : Min Stack

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#Stack #InterviewQuestion #Leetcode #Java #DSACourse #CollegeWallah #PWSkills #PhysicsWallah

[AnanyaShukla-zr]

1:14:57 in ques 5 it shoud be while(st.size()>0 && st.peek()<arr[i]){
to avoid error

[ShachiSinghal-fiuh]

I hope one day, I can also think whole logic on my own.

[starzenno]

1:27:38
public class NextGreatestElement {

private static int[] nextGreatest(int[] arr) {
int n = arr.length;
int[] res = new int[n];
Stack<Integer> st = new Stack<>();
res[n - 1] = -1;
st.push(arr[n - 1]);
for (int i = n - 2; i >= 0; i--) {
while (!st.isEmpty() && st.peek() < arr[i]) {
st.pop();
}
if (st.isEmpty()) res[i] = -1;
else res[i] = st.peek();
st.push(arr[i]);
}
return res;
}

public static void main(String[] args) {
int[] arr = {1, 5, 3, 2, 1, 6, 3, 4};
int[] res = nextGreatest(arr);

}
}

[tricksforsolving]

1:47:02 gfg
class Solution {
// Function to calculate the span of stock’s price for all n days.
public static int[] calculateSpan(int price[], int n) {
// Your code here
Stack<Integer> st=new Stack<>();
int res[]=new int[price.length];
res[0]=1;
st.push(0);
for(int i=1;i<price.length;i++){
while(st.size()>0 && price[st.peek()]<=price[i]){
st.pop();
}
if(st.size()==0){
res[i]=i+1;
}else{
res[i]=i-st.peek();
}
st.push(i);
}
return res;
}
}

[jeetukumar]

Sir please trees krana ab queqe ke baad, aapki wjh se DSA me mja aa rha h

[magicbullet]

Stock Span Problem 1:31:00
public static int[] stockSpan(int[] arr){
int[] res= new int[arr.length];
Stack<Integer> st = new Stack<>();
res[0]=1;
st.push(0);
for(int i=1;i<arr.length;i++){
while(!st.isEmpty() && arr[st.peek()]<arr[i]){
st.pop();
}
if(st.isEmpty()) res[i]=1;
else res[i]=i-st.peek();
st.push(i);
}
return res;
}

[prithbi]

MY SOLUTION OF 29:54 Q4 : Remove consecutive subsequences

private static int[] solve(int[] arr){
Stack<Integer> st = new Stack<>();
boolean need = false;
for(int i=0;i<arr.length;i++){
if(i==0){
st.push(arr[i]);
}else if(st.peek()!=arr[i] && need){
st.pop();
st.push(arr[i]);
need = false;

}else if(st.peek()!=arr[i] && !need){
st.push(arr[i]);
}else{
need = true;
}
}


// System.out.println(st);
int[] newArr = new int[st.size()];
for(int i=0;i<st.size();i++){
newArr[i] = st.get(i);
}

return newArr;

}

[Raj-ejej]

HW 2 solution
static int wrongPara(String s) {
Stack<Character> stack= new Stack<>();
for(int i=0;i<s.length();i++) {
char ch=s.charAt(i);
if(ch=='(') {
stack.push(ch);
}
else {
if(!stack.isEmpty() && stack.peek()=='(') {
stack.pop();

}
else {
stack.push(ch);
}
}

}

return stack.size();
}

[deevyanshutiwari]

Q5) In while condition write like this

-> while(st.size()>0 && st.peek()<arr[i])

so that you can avoid some error like exception thread

[VikrantSingh-sbml]

min space withput extra space also comes with space complexity with O(n);

[akashgautam]

int arr[] = {1, 2, 3, 3, 3, 3, 4, 4, 4, 2, 2, 5, 5, 7, 8} is not working. it's give ans only 1, 7, 8 but according to questions ans should be 1, 2, 7, 8.. Please Discuss in next lecture..

Lecture is awesome...❤
Please reduce the time gap between uploading two consecutive lectures...

[aryansrivastava]

Are koi c++ ki series bhi complete kra do

[SnehaTechTalks]

Sir etne hards questions interview me aate hai ?? kya

[PhysicsSwami]

hi sir, for you code on remove Consecutive subsequence which you have given at time stamp around 55-56 minutes in your lecture... if we put the input of array 123332445, then the solution should be 1225, but your code will give output 15.... WHY??... because after you pop 3 because of its repitition, your top of stack is 2 from before. then you are again going to the element 2 that lies after the 3's sequence. And your code compares it will top of stack via peek function and thus removes that 2 also. But according to logic it shouldn't be removed as it is not in a consecutive sequence of 2's... PLEASE THINK OVER IT AND LET ME KNOW. Anyways very nice presentation sir. Thanks.

[by_me]

h3
class Solution {
public boolean isValid(String s) {
Stack<Character> st=new Stack<>();
for(int i=0;i<s.length();i++) {
char ch=s.charAt(i);
if(ch=='('|| ch=='{' || ch=='[') st.push(ch);
else if(st.size()>0) {
char dh=st.peek();
if(ch==')') {
if(dh=='(') st.pop();
else return false;
}
if(ch==']') {
if(dh=='[') st.pop();
else return false;
}
if(ch=='}') {
if(dh=='{') st.pop();
else return false;
}
}
else return false;
}
if(st.size()>0) return false;
return true;
}
}

[keyurchandrikapure]

55:10 I tried coding it first and did the same mistake😂😅

[by_me]

HW3 solution
class Solution {
public boolean isValid(String s) {
Stack<Character> st=new Stack<>();
for(int i=0;i<s.length();i++) {
char ch=s.charAt(i);
if(ch=='('|| ch=='{' || ch=='[') st.push(ch);
if(st.size() == 0) return false;
if(ch==')') {
if(st.peek()=='(') st.pop();
else return false;
}
if(ch==']') {
if(st.peek()=='[') st.pop();
else return false;
}
if(ch=='}') {
if(st.peek()=='{') st.pop();
else return false;
}
}
if(st.size()>0) return false;
return true;
}
}

[NehaTyagi-sz]

Q3 sol=>public static boolean isValid(String s) {
Stack<Character> st = new Stack<>();
for (int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
if (ch == '(' || ch == '[' || ch == '{') {
st.push(ch);
}
else {
if (st.isEmpty()) {
return false;
}
else {
char top = st.pop();
if ((ch == ')' && top != '(') || (ch == ']' && top != '[') || (ch == '}' && top != '{')) {
return false;
}
}
}
}
return st.isEmpty();
}

[priyanshgupta]

Wrong Answer
Runtime: 0 ms
Case 1
Case 2
Input
nums =
[1, 2, 1]
Output
[2, -1, 1]
Expected
[2, -1, 2]
for nextgreaterelement2

[anassaif]

Why always directly jumping to solution! ??? Will never tell How to think ? What should be the approach??