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What is the time complexity of the optimal job scheduling algorithm that sorts jobs by end time and uses binary search with a DP array of size n?

medium🪤 Complexity Trap Q13 of Q15
Dynamic Programming: Knapsack - Maximum Profit in Job Scheduling
What is the time complexity of the optimal job scheduling algorithm that sorts jobs by end time and uses binary search with a DP array of size n?
AO(n log W) where W is the maximum end time
BO(n^2) due to nested loops over all jobs
CO(n) because each job is processed once
DO(n log n) due to sorting and binary searches for each job
Step-by-Step Solution
  1. Step 1: Identify sorting cost

    Sorting n jobs by end time costs O(n log n).
  2. Step 2: Analyze DP loop with binary search

    For each job, binary search among ends array costs O(log n), repeated n times -> O(n log n).
  3. Final Answer:

    Option D -> Option D
  4. Quick Check:

    Sorting + n binary searches dominate; no nested O(n^2) loops [OK]
Quick Trick: Sorting + binary search per job -> O(n log n) [OK]
Common Mistakes:
MISTAKES
  • Assuming O(n^2) due to DP
  • Ignoring binary search cost
  • Confusing max end time W with input size n
Trap Explanation:
PITFALL
  • O(n^2) looks plausible since DP often involves nested loops, but binary search reduces inner loop to log n.
Interviewer Note:
CONTEXT
  • Checks if candidate understands how binary search optimizes DP and distinguishes input size from value range.
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