Top 20 Array Interview Questions with Pattern Analysis

Arrays are the most versatile and foundational data structure in programming interviews. Almost every coding assessment features at least one problem involving array manipulation or pattern recognition.

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Essential Patterns to Identify

1. Two-Pointer Approach

When an array is sorted or requires pair comparisons from opposite ends, two pointers reduce time complexity from O(N^2) to O(N).

Example: Two Sum II (Sorted Input)

// C++ Implementation
vector<int> twoSum(vector<int>& numbers, int target) {
    int left = 0, right = numbers.size() - 1;
    while (left < right) {
        int sum = numbers[left] + numbers[right];
        if (sum == target) return {left + 1, right + 1};
        else if (sum < target) left++;
        else right--;
    }
    return {};
}

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2. Sliding Window Technique

Ideal for contiguous sub-array problems requiring max/min sum or substring constraints.

Example: Maximum Sum Subarray of Size K

int maxSubArrayOfSizeK(int k, const vector<int>& arr) {
    int maxSum = 0, windowSum = 0;
    int windowStart = 0;
    for (int windowEnd = 0; windowEnd < arr.size(); windowEnd++) {
        windowSum += arr[windowEnd];
        if (windowEnd >= k - 1) {
            maxSum = max(maxSum, windowSum);
            windowSum -= arr[windowStart];
            windowStart++;
        }
    }
    return maxSum;
}

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3. Kadane's Algorithm (Maximum Subarray Sum)

Kadane's algorithm computes the maximum sum contiguous subarray in linear O(N) time with O(1) space.

int maxSubArray(vector<int>& nums) {
    int maxSoFar = nums[0];
    int currentMax = nums[0];
    for (size_t i = 1; i < nums.size(); ++i) {
        currentMax = max(nums[i], currentMax + nums[i]);
        maxSoFar = max(maxSoFar, currentMax);
    }
    return maxSoFar;
}

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Top 20 Curated Array Problems for Interview Prep

1. Two Sum (Hash Map - O(N))
2. Best Time to Buy and Sell Stock (Single Pass - O(N))
3. Contains Duplicate (Hash Set - O(N))
4. Product of Array Except Self (Prefix & Suffix products - O(N))
5. Maximum Subarray (Kadane's Algorithm) (O(N))
6. Maximum Product Subarray (Dynamic tracking of min/max - O(N))
7. Find Minimum in Rotated Sorted Array (Binary Search - O(log N))
8. Search in Rotated Sorted Array (Binary Search - O(log N))
9. 3Sum (Sorting + Two Pointers - O(N^2))
10. Container With Most Water (Two Pointers - O(N))
11. Trapping Rain Water (Two Pointers / Monotonic Stack - O(N))
12. Merge Intervals (Sorting - O(N log N))
13. Insert Interval (Linear scan - O(N))
14. Non-overlapping Intervals (Greedy - O(N log N))
15. Rotate Image / 2D Matrix (Transpose & Reverse - O(N^2))
16. Set Matrix Zeroes (In-place markers - O(M*N))
17. Spiral Matrix (Boundary Traversal - O(M*N))
18. Subarray Sum Equals K (Prefix Sum + Hash Map - O(N))
19. Longest Consecutive Sequence (Hash Set - O(N))
20. Next Permutation (Single pass reverse scan - O(N))

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Best Practices During Interviews

• Clarify constraints: Ask about negative numbers, duplicates, and integer overflow edge cases.
• Start with brute-force: State the naive solution first, explain why it is slow, then optimize with patterns.
• Dry run edge cases: Always test empty arrays, single-element arrays, and all-duplicate arrays before submitting.