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Week 15: Arrays & Hash Maps
| Phase 6: Fast Track — Core Patterns
Day 1-2: Arrays & Hash Maps
3-4 hours
📚 Learning
Brute-force: nested loops O(n²) — try every pair
Optimal: hash map O(n) — single pass lookup
Brute-force: sort + compare neighbours O(n log n) or nested O(n²)
Optimal: hash set O(n) — single pass
Brute-force: sort both strings O(n log n)
Optimal: Counter / frequency array O(n)
Brute-force: compare all pairs of strings O(n² · k)
Optimal: sorted-string key in hash map O(n · k log k)
Brute-force: count frequency + sort O(n log n)
Optimal: bucket sort O(n)
Brute-force: nested loop multiply all except self O(n²)
Optimal: prefix/suffix products O(n) no division
🔗 LeetCode
1. Two Sum
Easy
217. Contains Duplicate
Easy
242. Valid Anagram
Easy
49. Group Anagrams
Medium
347. Top K Frequent Elements
Medium
238. Product of Array Except Self
Medium
Day 3-4: Two Pointers & Sliding Window
3-4 hours
📚 Learning
Brute-force: reverse + compare new string O(n) extra space
Optimal: two pointers inward O(n) O(1) space
Brute-force: triple nested loops O(n³)
Optimal: sort + two pointers O(n²), skip duplicates
Brute-force: check every pair of lines O(n²)
Optimal: two pointers from edges inward O(n)
Brute-force: check every buy-sell pair O(n²)
Optimal: track min price, single pass O(n)
Brute-force: check every substring O(n³)
Optimal: sliding window + set O(n)
🔗 LeetCode
125. Valid Palindrome
Easy
15. 3Sum
Medium
11. Container With Most Water
Medium
121. Best Time to Buy and Sell Stock
Easy
3. Longest Substring Without Repeating Characters
Medium
Day 5-7: Stacks & Binary Search
3-4 hours
📚 Learning
Stack approach: push open, pop on close, check match O(n)
Brute-force: scan stack for min O(n) per getMin call
Optimal: auxiliary min-stack O(1) getMin
Optimal: binary search O(log n)
Brute-force: linear scan O(n)
Optimal: modified binary search on rotated array O(log n)
🔗 LeetCode
20. Valid Parentheses
Easy
155. Min Stack
Medium
704. Binary Search
Easy
33. Search in Rotated Sorted Array
Medium