Lists Checkpoint

Optional

20 min · Beginner

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A review of this module. Skip it if you feel confident; the next module does not need it.

Fix the mistakes

The slips that cost marks in this module. Spot each one, then check the fix.

Test your mastery of Python lists with fundamental technical coding round problems frequently asked in placement exams.

Finding the second largest element in O(N)

Track the largest and second largest values in a single pass without sorting.

Python

1nums = [12, 35, 1, 10, 34, 1]2first = second = float('-inf')3for x in nums:4    if x > first:5        second = first6        first = x7    elif x > second and x != first:8        second = x9print(f"Second largest: {second}")

Example 1 · Largest value and where it is

Python

sales = [120, 340, 90, 340, 15]top = max(sales)print(top)print(sales.index(top))print(sales.count(top))

Output

340
1
2

What this code does

max finds the biggest value in one pass.
index gives the first position of that value, and count says it appears twice.

This single-pass algorithm operates in O(N) time and O(1) space, outperforming O(N log N) sorting.

List rotation by K steps

Rotate a list by k steps using slice concatenation.

Python

nums = [1, 2, 3, 4, 5]k = 2k = k % len(nums)rotated = nums[-k:] + nums[:-k]print(f"Rotated by {k}: {rotated}")

Example 2 · Removing repeats from a sorted list

Python

1arr = [1, 1, 2, 2, 3, 4, 4]2# Remove duplicates preserving order3unique = []4for x in arr:5    if not unique or unique[-1] != x:6        unique.append(x)7print("Unique items:", unique)

Output

Unique items: [1, 2, 3, 4]

What this code does

Iterates through sorted array.
Appends element only if it differs from the last added element.
Produces clean unique list [1, 2, 3, 4].

Example 3 · Running totals

Python

1daily = [5, 3, 8, 2]2running = []3total = 04for amount in daily:5    total += amount6    running.append(total)7print(running)

Output

[5, 8, 16, 18]

What this code does

Each round adds the new amount to total and stores the result.
The new list is built item by item, so daily is never changed.

Slicing nums[-2:] gives [4, 5], and nums[:-2] gives [1, 2, 3]. Concatenating them produces [4, 5, 1, 2, 3].

Finish up

Mark complete & continue

Next: Must-do problems: Lists

1nums = [12, 35, 1, 10, 34, 1]2first = second = float('-inf')3for x in nums:4 if x > first:5 second = first6 first = x7 elif x > second and x != first:8 second = x9print(f"Second largest: {second}")