What are the set of functions that are to be executed to get the following…
2023
What are the set of functions that are to be executed to get the following output?
cat
- A.
push(c, s); push(a, s); push(t, s); pop(s); pop(s); pop(s);
- B.
push(c, s); pop(s); push(a, s); pop(s); push(t, s); pop(s);
- C.
pop(c); pop(a); pop(t);
- D.
push(c, s); push(a, s); pop(t);
Attempted by 27 students.
Show answer & explanation
Correct answer: B
Concept
A stack supports only two operations, push and pop, and follows the Last-In-First-Out (LIFO) rule — whichever element was pushed most recently is the one returned by the very next pop. So the order in which a sequence of pop() calls emits characters depends entirely on how the push and pop calls are interleaved: pushing several elements before popping any of them reverses their retrieval order, while popping immediately after each individual push preserves the original order.
Application
push(c, s) places c on the stack; pop(s) runs immediately and removes/outputs c, leaving the stack empty.
push(a, s) places a on the stack; pop(s) runs immediately and removes/outputs a, leaving the stack empty.
push(t, s) places t on the stack; pop(s) runs immediately and removes/outputs t, leaving the stack empty.
The characters are produced in the order c, a, t — exactly the required output "cat".
Cross-check
Pushing all three characters first and popping only afterward reverses the order under LIFO, producing "tac" instead of "cat".
A pop() call issued before any push has nothing on the stack to remove, since the stack starts empty.
A sequence that pushes only two of the three characters can never emit the missing third character via pop, since pop only returns what was previously pushed.