greedy invariant Pattern
Pattern hubs are for building transferable solving frames. Learn the recognition signals first, then drill state definition, update rules, and edge explanation until the pattern feels stable.
Pattern brief
Recognize first
The candidate demonstrates a good understanding of greedy algorithms by proposing a solution based on the greedy choice paradigm.
Solve rhythm
State the active state and invariant first, explain how each update preserves them, then pressure-test with counterexamples.
Most common miss
Failing to consider the case where the total gas is less than the total cost, which leads to an incorrect answer.
Recognition signals
- The candidate demonstrates a good understanding of greedy algorithms by proposing a solution based on the greedy choice paradigm.
- The candidate explains invariant validation correctly, ensuring that they track gas levels and station indices effectively throughout the solution.
- Look for a clear understanding of greedy algorithms and how to implement the left-to-right and right-to-left passes.
Solve flow
- 1. Define the active state/window.
- 2. Update state while preserving invariants.
- 3. Validate with edge-heavy examples.
Common misses
- Failing to consider the case where the total gas is less than the total cost, which leads to an incorrect answer.
- Failing to correctly handle the right-to-left pass, potentially leading to incorrect candy assignments.
- Overlooking the need for a custom sorting function for concatenation comparisons.
Recommended Ladder
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