Time complexity:

O(n)

This is a very important array interview problem because it tests whether you can optimize a seemingly expensive problem.

🔹 14. What is the difference between a subarray and a subsequence?

Answer:

A subarray must contain contiguous elements.

Example:

[2, 3, 4]

is a subarray of:

[1, 2, 3, 4, 5]

A subsequence does not need to be contiguous, but the original order must be maintained.

Example:

[1, 3, 5]

is a subsequence of:

[1, 2, 3, 4, 5]

but it is not a contiguous subarray.

This distinction becomes extremely important in Dynamic Programming problems such as Longest Common Subsequence.

🔹 15. How do you rotate an array?

Example:

Input:

[1, 2, 3, 4, 5]

Rotate right by 2:

[4, 5, 1, 2, 3]

One efficient approach uses the reversal technique.

Conceptually:

Reverse entire array

Reverse first k elements

Reverse remaining elements

For:

[1, 2, 3, 4, 5]

rotate right by "2":

Reverse all:

[5, 4, 3, 2, 1]

Reverse first 2:

[4, 5, 3, 2, 1]

Reverse remaining:

[4, 5, 1, 2, 3]

This technique can achieve:

Time: O(n)

Space: O(1)

Learn to recognize patterns:

✔ Two Pointers

✔ Sliding Window

✔ Prefix Sum

✔ HashMap / HashSet

✔ Binary Search

✔ Sorting

✔ Kadane's Algorithm

✔ In-place modification

Once you recognize the pattern, many seemingly different array problems become much easier to solve.

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