[Solved] Consider the following buggy code for reorganizing a string. Which... Which line contains the subtle bug that can cause the function to return an invalid string with adjacent identical characters? | Greedy Algorithms

Greedy Algorithms - Reorganize String (No Two Adjacent Same)

Consider the following buggy code for reorganizing a string. Which line contains the subtle bug that can cause the function to return an invalid string with adjacent identical characters?

import heapq
from collections import Counter

def reorganizeString(s: str) -> str:
    freq = Counter(s)
    max_heap = [(-count, ch) for ch, count in freq.items()]
    heapq.heapify(max_heap)
    prev_count, prev_char = 0, ''
    result = []

    while max_heap:
        count, ch = heapq.heappop(max_heap)
        result.append(ch)
        # Bug: missing check for impossible case
        if prev_count < 0:
            heapq.heappush(max_heap, (prev_count, prev_char))
        prev_count, prev_char = count + 1, ch

    res_str = ''.join(result)
    if len(res_str) != len(s):
        return ""
    return res_str

AMissing check before the while loop to verify if max frequency exceeds (n+1)/2

BLine where prev_count and prev_char are updated after appending

CLine where the previous character is pushed back into the heap

DLine where max_heap is initialized with negative counts

Step-by-Step Solution

Solution:

Step 1: Identify missing validation
The code lacks a check before the loop to verify if the most frequent character count exceeds (n+1)/2, which makes rearrangement impossible.
Step 2: Understand consequences
Without this check, the algorithm may produce invalid strings with adjacent duplicates or fail silently.
Final Answer:
Option A -> Option A
Quick Check:
Adding this check prevents impossible cases early [OK]

Quick Trick: Check max frequency before processing [OK]

Common Mistakes:

MISTAKES

Forgetting impossible case check
Misplacing heap pushback logic
Incorrect prev_count update

Trap Explanation:

PITFALL

Candidates often overlook the feasibility check, assuming heap logic suffices for all inputs.

Interviewer Note:

CONTEXT

Tests if candidate knows necessary preconditions for greedy heap approach correctness.

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More Greedy Algorithms Quizzes

Consider the following buggy code for reorganizing a string. Which line contains the subtle bug that can cause the function to return an invalid string with adjacent identical characters?

Step 1: Identify missing validation

Step 2: Understand consequences

Final Answer:

Quick Check:

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