Top 10 most array problems for FAANG interview

Here are the Top 10 most common and high-impact array problems that frequently appear in FAANG interviews (Google, Amazon, Meta, Apple, Netflix, Microsoft, etc.), with difficulty levels, patterns, and reasoning focus.

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🧠 Top 10 Array Problems for FAANG Interviews

#	Problem	Pattern / Concept	Key Skills Tested	Typical Difficulty
1️⃣	Two Sum	Hashing	Hash map lookup to find complements	Easy
2️⃣	Best Time to Buy and Sell Stock	Kadane’s variation / Sliding window	Greedy profit calculation	Easy–Medium
3️⃣	Product of Array Except Self	Prefix/Suffix product	Avoiding division, O(n) solution	Medium
4️⃣	Maximum Subarray Sum (Kadane’s Algorithm)	Dynamic programming	Running sum tracking	Medium
5️⃣	Merge Intervals	Sorting + Merging	Overlap handling logic	Medium
6️⃣	Dutch National Flag / Sort Colors	Two pointers	In-place sorting (0,1,2)	Medium
7️⃣	Container With Most Water	Two pointers	Area optimization	Medium
8️⃣	3Sum / 4Sum	Sorting + Two pointers	Combination logic & duplicates handling	Medium–Hard
9️⃣	Find Duplicate Number	Floyd’s Cycle Detection / Hashing	Detecting cycle or repeated element	Medium
🔟	Trapping Rain Water	Two pointers / Stack	Area computation using boundary heights	Hard

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⚙️ Quick Pattern Summary

Pattern	Example Problems	Focus
Two Pointers	Two Sum (sorted), 3Sum, Container with Most Water, Sort Colors	Moving inward, optimizing space
Prefix/Suffix / Kadane	Max Subarray, Product Except Self, Buy-Sell Stock	Subarray accumulation, O(1) space
Sorting-based	Merge Intervals, 3Sum, Meeting Rooms	Handling order & overlap
Hash Map	Two Sum, Subarray Sum = K, Find Duplicate	Constant time lookups
Stack	Trapping Rain Water, Next Greater Element	Tracking elements with conditions

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🏁 Bonus Problems (Honorable Mentions)

Problem	Why Important
Rotate Array	In-place reversal technique
Find Missing Number	XOR or arithmetic trick
Subarray Sum Equals K	Prefix sum + hash map
Maximum Product Subarray	DP with positive/negative tracking
Longest Consecutive Sequence	Hash set O(n) logic

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🔍 Tips for Array Questions

1. Always identify the pattern first (sliding window, two pointers, prefix sum, etc.).
2. Focus on time complexity → most answers must be O(n) or O(n log n).
3. Write edge cases mentally — empty array, duplicates, negative numbers.
4. Be ready to code in any of C++ / Java / Python.

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🧠 Top 10 Array Problems — Quick Strategy & Code

#	Problem	One-Liner Strategy	Python Code Snippet
1️⃣	Two Sum	Use a hash map to find the complement target - num in O(n).	python def twoSum(nums, target): seen = {} for i, n in enumerate(nums): if target - n in seen: return [seen[target - n], i] seen[n] = i
2️⃣	Best Time to Buy & Sell Stock	Track min price & max profit on each iteration.	python def maxProfit(prices): minp, profit = float('inf'), 0 for p in prices: minp = min(minp, p); profit = max(profit, p - minp) return profit
3️⃣	Product of Array Except Self	Compute prefix * suffix products in O(n) w/o division.	python def productExceptSelf(nums): res = [1]*len(nums) left = 1 for i in range(len(nums)): res[i] = left; left *= nums[i] right = 1 for i in range(len(nums)-1,-1,-1): res[i] *= right; right *= nums[i] return res
4️⃣	Maximum Subarray (Kadane’s)	Track running max; reset when negative.	python def maxSubArray(nums): best = cur = nums[0] for n in nums[1:]: cur = max(n, cur + n); best = max(best, cur) return best
5️⃣	Merge Intervals	Sort by start; merge overlapping intervals.	python def merge(intervals): intervals.sort() res = [intervals[0]] for s, e in intervals[1:]: if s <= res[-1][1]: res[-1][1] = max(res[-1][1], e) else: res.append([s, e]) return res
6️⃣	Sort Colors (Dutch Flag)	Use 3 pointers (low, mid, high).	python def sortColors(nums): l, m, r = 0, 0, len(nums)-1 while m <= r: if nums[m]==0: nums[l], nums[m]=nums[m],nums[l]; l+=1; m+=1 elif nums[m]==2: nums[m],nums[r]=nums[r],nums[m]; r-=1 else: m+=1
7️⃣	Container With Most Water	Two pointers, move shorter line inward.	python def maxArea(h): l, r, area = 0, len(h)-1, 0 while l<r: area = max(area, min(h[l],h[r])*(r-l)) if h[l]<h[r]: l+=1 else: r-=1 return area
8️⃣	3Sum	Sort + fix one element + two-pointer search.	python def threeSum(nums): nums.sort(); res=[] for i,a in enumerate(nums): if i>0 and a==nums[i-1]: continue l,r=i+1,len(nums)-1 while l<r: s=a+nums[l]+nums[r] if s<0:l+=1 elif s>0:r-=1 else: res.append([a,nums[l],nums[r]]); l+=1; while l<r and nums[l]==nums[l-1]: l+=1 return res
9️⃣	Find Duplicate Number	Use Floyd’s cycle detection (linked list analogy).	python def findDuplicate(nums): slow, fast = nums[0], nums[0] while True: slow = nums[slow]; fast = nums[nums[fast]]; if slow==fast: break slow = nums[0] while slow!=fast: slow = nums[slow]; fast = nums[fast] return slow
🔟	Trapping Rain Water	Use two pointers, track left/right max walls.	python def trap(h): l,r=0,len(h)-1; lm=rm=water=0 while l<r: if h[l]<h[r]: lm=max(lm,h[l]); water+=lm-h[l]; l+=1 else: rm=max(rm,h[r]); water+=rm-h[r]; r-=1 return water

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⚙️ Tips for Mastery

Focus Area	How to Practice
Two Pointers	Practice 2Sum, 3Sum, Container with Water, Sort Colors
Sliding Window	Try “Longest Substring Without Repeating Characters”
Prefix/Suffix	Product Except Self, Subarray Sum = K
Hashing	Two Sum, Subarray Sum problems
Greedy + DP	Kadane, Buy/Sell Stock

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