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Given the intermediate stack state ['1', '2', '1'] and k=1 after processing the first 6 digits of the input number, what could be the original input number prefix?

hard🔄 Reverse Engineer Q9 of Q15

Greedy Algorithms - Remove K Digits (Smallest Number)

Given the intermediate stack state ['1', '2', '1'] and k=1 after processing the first 6 digits of the input number, what could be the original input number prefix?

A"143221"

B"123211"

C"121219"

D"132211"

Step-by-Step Solution

Solution:

Step 1: Recognize stack state after removals
Stack ['1','2','1'] with k=1 means one removal left after processing 6 digits.
Step 2: Match known example input prefix
Input "143221" processed with k=3 leads to this stack and k=1 remaining.
Final Answer:
Option A -> Option A
Quick Check:
Matches known intermediate state from example [OK]

Quick Trick: Stack state and k track removals and input prefix [OK]

Common Mistakes:

MISTAKES

Confusing stack content with final output
Ignoring remaining k

Trap Explanation:

PITFALL

Candidates often fail to reason backwards from stack and k to input prefix correctly.

Interviewer Note:

CONTEXT

Tests deep understanding of algorithm state and reverse reasoning.

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