code-optimizer-skill
📖 技能介绍
name: code-optimizer description: Use when code has performance issues, slow execution, high memory usage, nested loops, timeout errors, TLE, or fails performance benchmarks. Also use for algorithm complexity analysis, data structure optimization, or when user mentions Big O, O(n^2), time/space complexity, performance tuning, optimize, speed up code in C++, Java, or Python.
Code Optimizer Skill
Overview
Transform code to reduce time and space complexity while maintaining correctness. Focus on proven optimization patterns across C++, Java, and Python.
Core principle: Measurably improve complexity, verify behavior unchanged.
When to Use
Use for ANY performance-related work: - Slow code, timeouts, memory issues - Nested loops, repeated lookups - Algorithm/data structure selection - Complexity analysis requests
Use ESPECIALLY when: - User mentions "optimize", "speed up", "performance" - Code has O(N^2) or worse patterns - Timeout errors (TLE) in competitive programming - Memory pressure or GC overhead issues
Quick Reference
| Symptom | Likely Issue | Quick Fix |
|---|---|---|
| Nested loops over same data | O(N^2) | Hash table or two pointers |
| Lookup in unsorted list | O(N) per lookup | Convert to set/hash |
| String concat in loop | O(N^2) memory churn | StringBuilder/reserve |
| Repeated sorting | O(N log N) each | Sort once, cache sorted |
| Recursion explosion | Exponential | Memoization/DP |
| Vector growth without reserve | Multiple allocations | Pre-allocate capacity |
Optimization Workflow
Follow these steps for every optimization:
Optimization Progress:
- [ ] Step 1: Identify current complexity
- [ ] Step 2: Analyze bottleneck source
- [ ] Step 3: Select optimization strategy
- [ ] Step 4: Implement minimal change
- [ ] Step 5: Verify complexity improvement
- [ ] Step 6: Confirm behavior unchanged
Step 1: Identify current complexity - Count loop nesting depth - Identify data structure operations (lookup, insert, iterate) - Calculate worst-case operations count - Express as Big O notation
Step 2: Analyze bottleneck source
| Pattern | Likely Bottleneck |
|---|---|
| Nested iteration over same data | O(N^2) from double loop |
| Lookup in unsorted list/array | O(N) per lookup |
| Repeated sorting | O(N log N) per sort |
| String concatenation in loop | O(N^2) from repeated copies |
| Recursion without memoization | Exponential branching |
| Large object creation in loop | Memory pressure + GC overhead |
Step 3: Select optimization strategy
See Algorithm Selection Decision Tree below.
Step 4: Implement minimal change - ONE change at a time - No bundled refactoring - No "while I'm here" improvements
Step 5: Verify complexity improvement - Recalculate Big O - Confirm improvement target achieved - Test with increasing input sizes
Step 6: Confirm behavior unchanged - Run existing tests - Manual verification of edge cases - Output comparison before/after
Algorithm Selection Decision Tree
digraph algorithm_selection {
rankdir=TB;
"Nested loops identified?" [shape=diamond];
"Lookup operation?" [shape=diamond];
"Hash Table\n(O(1) lookup)" [shape=box, style=filled, fillcolor="#ccffcc"];
"Range query?" [shape=diamond];
"Tree/Heap\n(O(log N) query)" [shape=box, style=filled, fillcolor="#ccffcc"];
"Sliding pattern?" [shape=diamond];
"Two Pointers/Window\n(O(N))" [shape=box, style=filled, fillcolor="#ccffcc"];
"Subproblem reuse?" [shape=diamond];
"DP/Memoization\n(Polynomial)" [shape=box, style=filled, fillcolor="#ccffcc"];
"Ordering needed?" [shape=diamond];
"Pre-sort once\n(O(N log N))" [shape=box, style=filled, fillcolor="#ccffcc"];
"Keep nested loops\n(document constraint)" [shape=box, style=filled, fillcolor="#ffcccc"];
"Nested loops identified?" -> "Lookup operation?";
"Lookup operation?" -> "Hash Table\n(O(1) lookup)" [label="yes"];
"Lookup operation?" -> "Range query?";
"Range query?" -> "Tree/Heap\n(O(log N) query)" [label="yes"];
"Range query?" -> "Sliding pattern?";
"Sliding pattern?" -> "Two Pointers/Window\n(O(N))" [label="yes"];
"Sliding pattern?" -> "Subproblem reuse?";
"Subproblem reuse?" -> "DP/Memoization\n(Polynomial)" [label="yes"];
"Subproblem reuse?" -> "Ordering needed?";
"Ordering needed?" -> "Pre-sort once\n(O(N log N))" [label="yes"];
"Ordering needed?" -> "Keep nested loops\n(document constraint)" [label="no - inherent O(N^2)"];
}
Decision explanations:
| Decision | Condition | Strategy | Result |
|---|---|---|---|
| Hash Table | Repeated lookup needed | Replace list lookup with set/map | O(N) -> O(1) per lookup |
| Tree/Heap | Need min/max/range queries | Use ordered structure | O(N) -> O(log N) per query |
| Two Pointers | Contiguous range pattern | Replace nested loop with pointers | O(N^2) -> O(N) |
| DP/Memo | Same subproblems calculated | Cache results | O(2^N) -> O(N) or O(N^2) |
| Pre-sort | Ordering enables simpler logic | Sort once, use sorted property | Varies |
Common Anti-Patterns Summary
| Anti-Pattern | Detection Signal | Impact | Fix |
|---|---|---|---|
| List lookup in loop | in list inside for |
O(N^2) | Use set() |
| String concat in loop | += on strings in loop |
O(N^2) memory | StringBuilder |
| Unreserved growth | push_back/append only |
Multiple allocations | .reserve(N) |
| Repeated sorting | sort() inside loop |
O(N log N) each | Sort once |
| Recursion explosion | Recursive call, no memo | O(2^N) | Memoization |
Full anti-pattern details: See references/anti-patterns.md
Optimization patterns library: See references/optimization-patterns.md
Complete examples: See references/examples.md
Complexity formulas: See references/complexity-formulas.md
Language-Specific Quick Tips
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| Language | Key Optimizations |
|---|---|
| C++ | reserve(), unordered_map, move semantics, std::span, range-based algorithms |
| Java | StringBuilder with capacity, primitive streams (IntStream), ArrayDeque |
| Python | List comprehensions, collections module, generators, set() for membership |
Red Flags - STOP and Investigate
If you catch yourself thinking: - "Just make it faster" without measuring - "Optimization is obvious" without analysis - "Tests still pass so it's fine" without complexity check - "I'll optimize later" in hot path code - Skip verification after optimization
All of these mean: STOP. Follow the workflow.
Common Rationalizations
| Excuse | Reality |
|---|---|
| "Issue is simple, no need to analyze" | Simple issues have root causes too. Calculate first. |
| "Just try this fix first" | First fix sets the pattern. Do it right from the start. |
| "Tests pass so optimization works" | Tests pass != complexity improved. Measure. |
| "Multiple optimizations at once" | Can't isolate what worked. One change only. |
| "I'll measure later" | Measurement proves optimization. Do it now. |
When NOT to Optimize
- Code already O(N) or O(N log N) with acceptable performance
- Premature optimization in prototype/exploration
- Hot path identified but not yet measured
- Complexity inherent to problem (e.g., all pairs must be checked)
Decision Logic: When to Ask Questions
DO NOT guess constraints for critical performance code. Ask: - "What is the maximum input size?" - "Is this latency-sensitive or throughput-sensitive?" - "Are there memory limits to consider?" - "What is the target time/space complexity?"
Performance Verification
Before claiming optimization success:
Language-specific profiling commands:
C++:
g++ -O2 -pg solution.cpp -o solution
./solution input.txt
gprof solution gmon.out > analysis.txt
Java:
java -Xprof Solution
Python:
python -m cProfile -s tottime solution.py
Scaling test:
for size in 100 1000 10000 100000; do
time ./solution input_${size}.txt
done
# Expected: Linear scaling for O(N), log-linear for O(N log N)
# Warning: Quadratic scaling (2x size -> 4x time) indicates O(N^2)
🤖 AI 评测
这个技能质量优秀，内容全面且实用。它提供了清晰的优化步骤指引、丰富的代码示例（涵盖三种主流语言），以及详细的反模式识别指南。优点是覆盖面广、示例充分、流程规范；主要不足是缺少自动化的验证测试和质量保障机制。适合需要提升代码性能或学习算法优化的开发者使用。
