Practice Question (Gate 1991)

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This short GATE 1991 practice question asks students to trace a stack operation sequence and identify the correct popped-value order. The on-screen prompt reads “Choose the correct alternatives (more than one may be correct) and write the corresponding letters only,” followed by the operation list: PUSH(10), PUSH(20), POP, PUSH(10), PUSH(20), POP, POP, POP, PUSH(20), POP. Four options are shown: a) 20,10,20,10,20; b) 20,20,10,10,20; c) 10,20,20,10,20; d) 20,20,10,20,10. The instructor solves it by drawing a vertical stack container and updating its state after each push or pop, recording the values removed. The traced popped sequence is 20, 20, 10, 10, 20, which matches option (b), marked with a green checkmark. The lesson emphasizes LIFO behavior: the most recently pushed item is popped first, and visualizing stack contents step by step prevents mismatches with the answer choices.

Chapters

1. 0:00 – 1:47 00:00-01:47

   The clip presents a GATE 1991 stack question with the on-screen instruction “Choose the correct alternatives (more than one may be correct)” and the operation sequence “PUSH(10), PUSH(20), POP, PUSH(10), PUSH(20), POP, POP, POP, PUSH(20), POP.” Options a–d list possible popped sequences. The instructor draws a vertical stack container, pushes 10 then 20, pops the top value 20, continues through the remaining operations, and writes the popped sequence “20 20 10 10” plus the final pop, matching option (b) “20,20,10,10,20,” which is checked in green.

The core concept is stack LIFO tracing. The method shown is to maintain a visible stack state, apply each operation in order, and record every POP result. The worked example starts with an empty stack: PUSH(10) gives [10]; PUSH(20) gives [10,20]; POP removes 20; PUSH(10) and PUSH(20) rebuild the top; subsequent POPs remove 20, then 10, then 10; finally PUSH(20) and POP remove 20. The resulting sequence is 20,20,10,10,20, so option (b) is correct. For exam revision, students should practice writing the stack after each step and comparing only POP outputs to the options.