Data Structures - lesson 1 | HackIndia Skills

Data Structures - lesson 1 | HackIndia Skills | HackIndia

Example

Small cost table

Add a note like this in your repo. It is simple enough to revise before interviews.

Do not memorise blindly. Use it to explain trade-offs.

Data structure | Fast at | Usually costs
Array/List | index access, append | lookup by value is O(n)
Hash map | lookup by key, count, last seen | extra space O(n)
Hash set | membership check, unique values | extra space O(n)
Stack | last-in-first-out problems | only top is easy
Queue | first-in-first-out problems | only front/back are easy
Heap | repeated min/max | not full sorted order
Tree/BST | ordered structure | balance matters
Graph | relationships and paths | traversal needs visited tracking

Example

Worked choice

Problem: find if an array has any duplicate value.

Slow thought: compare every pair. That takes O(n²). Better thought: keep seen values in a set. Each membership check is usually O(1), so total time is O(n), with O(n) extra space.

def has_duplicate(nums):
    seen = set()
    for x in nums:
        if x in seen:
            return True
        seen.add(x)
    return False


def test_has_duplicate():
    assert has_duplicate([1, 2, 3, 2]) is True
    assert has_duplicate([1, 2, 3, 4]) is False
    assert has_duplicate([]) is False

Pick by cost first

One rule

Cost means operations

Ask these first

Small cost table

Worked choice

Common mistake

Use AI carefully

Do this now

Quick check