ISRO CSE 2008 | Question: 70

Question

Stack A has the entries a, b, c (with a on top). Stack B is empty. An entry popped out of stack A can be printed immediately or pushed to stack B. An entry popped out of the stack B can be only be printed. In this arrangement, which of the following permutations of a, b, c are not possible?

• A. b a c
• B. b c a
• C. c a b
• D. a b c

Answer 1

ans is C  c a b is not possible

Comments

can you please explain it why it is so..?

---

initially stack A will have a b c with a on top and empty stack B now if we pop from stack A a will come out and then b and then c these a b c can either be printed or pushed in B and whatever we pop from b can only be printed now one possibility is a is popped and pushed in B then b is popped from A and printed then a is popped and printed and finally c is popped and printed this will give bac which is option A. second possibility is a is popped and pushed in B then b is popped from A and printed then c is popped from A and printed and a is popped from B and printed which gives bca (Option B) . another possible sequence is unstack A which gives a b c (Option D) Option C is not possible as to print c first a and b must be pushed into B and when we pop b will come out before a which will give c b a

---

komal see here

stack A

a

b

c

 

stack B

for option a) sequence is...

POP A(stack)///////it will pop 'a' as it is on top

PUSH B(stack) /////// it will push 'a' into stack B

POP A  //////it will pop 'b' as it is on top

print .....it will print 'b'///////as POP from stack A can be printed immediately

POP B

print/////// it will print 'a'

POP A

print it will print 'c.....so we get 'bac'  as permuatation...

option (b)   similarly  POP A, PUSH B,POP A,print////b,POP A,print////c, POP B,print///a........bca

option(d) POP A,print///a, POP A,print///b, POP A,print///c.........abc printed

option(c)....this sequence is not possible...bcoz to print 'c' u need  to pop first 'a' then insert inside stack B and again..pop A...and insert inside STACK B..then u can print 'c'.. but...now u cannot..print 'a' ..coz 'b' is on top in STACK B..and in STACK B if u pop u have print it neccessarily...so 'b' has to printed..first..then only u can print 'a'....cab...not possible

---

must provide explanation....only answers are also available on other sites////////...selectors...must see this before selecting answers...

Answer 2

CAB is also possible

Here is how

First pop C from stack a and push it to stack b

Then pop C from stack b and print

Then pop 2 times from stack a and push to stack b Then pop the rest and print

The resulting sequence is cab

Comments

@Sarionic But in a stack, we can only pop the top element. Since ‘c’ is at the bottom of Stack A, we have to pop ‘a' and ‘b’ first. After popping them, they will go into Stack B (with ‘b’ on top of ‘a’). When we print from Stack B, ‘b’ will come before ‘a’, so the order CAB is not possible.

---

Yes you are right 👍 I think non of the options are correct

---

@IT_021_Manish_Kumar no, only option c is not possible

check this comment: https://gateoverflow.in/50138/isro-cse-2008-question-70?show=55339#c55339