Blind 75 — Array Pattern Guide

How to Identify an “Array” Problem

Interview Triggers

• Input is a single 1-D integer array (often nums[])
• Asks for an index pair / triplet / subarray that satisfies a condition (sum, product, max, etc.)
• Words like “contiguous”, “subarray”, “pair”, “k-th”, “rotated”
• Asks for min / max in a sequence
• Asks to search a sorted (or rotated sorted) array

Which Sub-Pattern Does It Belong To?

If the prompt says…	Use this sub-pattern	Example LC
“Find two numbers that sum to target”	HashMap complement	1
“Maximum profit from one buy/sell”	Single pass + min tracker	121
“Are there duplicates?”	HashSet	217
“Product of every element except itself” (no division)	Prefix × Suffix products	238
“Maximum sum of any contiguous subarray”	Kadane’s algorithm	53
“Maximum product of any contiguous subarray”	Track min AND max	152
“Find min in rotated sorted array”	Modified binary search	153
“Search target in rotated sorted array”	Modified binary search	33
“Find triplets that sum to zero”	Sort + two pointers (fix one)	15
“Maximize water/area between two lines”	Two pointers from edges	11

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The Decision Tree

ARRAY PROBLEM
│
├─ Sorted (or rotated sorted)?
│   ├─ Search / find element  → Binary Search (LC 33, 153)
│   └─ Pair/triplet sum       → Two Pointers (LC 15, 11)
│
├─ Need O(1) lookup of value/complement?
│   ├─ "Have I seen X?"       → HashSet (LC 217)
│   └─ "Pair that sums to T"  → HashMap value→index (LC 1)
│
├─ Asking about contiguous subarrays?
│   ├─ Sum (max/min)          → Kadane's (LC 53)
│   ├─ Product (max)          → Kadane variant (LC 152)
│   └─ Fixed window K         → Sliding window
│
├─ "Except itself" / running aggregate?
│   └─ Prefix + Suffix arrays (LC 238)
│
└─ Greedy / single-pass tracker?
    └─ LC 121 (min so far), LC 11 (shrink window)

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Bread & Butter Problems (Must-Know)

#	Problem	LC #	Difficulty	Sub-Pattern
1	Two Sum	1	Easy	HashMap complement
2	Best Time to Buy and Sell Stock	121	Easy	Min tracker + greedy diff
3	Contains Duplicate	217	Easy	HashSet
4	Maximum Subarray	53	Medium	Kadane’s
5	Product of Array Except Self	238	Medium	Prefix × Suffix products

FAANG “Aha!” Problems

#	Problem	LC #	Difficulty	Sub-Pattern
1	Maximum Product Subarray	152	Medium	Track min AND max
2	Find Min in Rotated Sorted Array	153	Medium	Modified binary search
3	Search in Rotated Sorted Array	33	Medium	Modified binary search
4	3Sum	15	Medium	Sort + two pointers
5	Container With Most Water	11	Medium	Two pointers from edges

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Java Implementation Tips

• Use int[] not List<Integer> whenever the size is fixed — avoids autoboxing overhead.
• Arrays.sort(int[]) uses Dual-Pivot Quicksort (O(n log n)) — safe for primitives.
• Arrays.sort(Object[]) uses Timsort (stable). Use this if you need stable order.
• For HashMap-keyed lookups, prefer Map.getOrDefault(key, 0) over containsKey + get.
• For “max so far / min so far” trackers, init with Integer.MAX_VALUE / Integer.MIN_VALUE.

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Senior Mental Trigger

“Sorted? → Binary search or two pointers. Unsorted? → HashMap. Contiguous? → Kadane or sliding window.”