Greedy Algorithms - Wiggle Subsequence
Given the following partial state of the up and down arrays after processing the input array:
up = [1, 2, 2, 3, 3]
down = [1, 1, 3, 3, 4]
What could be the original input array?
Given the following partial state of the up and down arrays after processing the input array: up = [1, 2, 2, 3, 3] down = [1, 1, 3, 3, 4] What could be the original input array?
hard🔄 Reverse Engineer Q9 of Q15
Step-by-Step Solution
Solution:
Step 1: Understand up/down arrays meaning
up[i] = length of longest wiggle subsequence ending with an upward wiggle at i; down[i] similarly for downward.Step 2: Match pattern to input
Values suggest alternating ups and downs with lengths matching known example [1,7,4,9,2].Final Answer:
Option A -> Option AQuick Check:
Only [1, 7, 4, 9, 2] matches wiggle subsequence lengths given [OK]
Quick Trick: Match up/down arrays to known wiggle subsequence patterns [OK]
Common Mistakes:
MISTAKES- Confusing arrays with input values
- Ignoring meaning of up/down arrays
Trap Explanation:
PITFALL- Candidates often fail to reverse engineer from DP arrays to input sequence.
Interviewer Note:
CONTEXT- Tests deep understanding of DP state and ability to reason backwards.
Master "Wiggle Subsequence" in Greedy Algorithms
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