Algorithms - lesson 3 | HackIndia Skills

Algorithms - lesson 3 | HackIndia Skills | HackIndia

Example

Example solution

Problem: Given a sorted array and a target, return whether any two numbers add to target.

This is a good repo problem because the brute force is easy to explain, and the pointer improvement is clear.

def has_pair_sum(nums, target):
    left = 0
    right = len(nums) - 1

    while left < right:
        current = nums[left] + nums[right]

        if current == target:
            return True
        if current < target:
            left += 1
        else:
            right -= 1

    return False


print(has_pair_sum([1, 2, 4, 6, 9], 10))  # True: 1 + 9 or 4 + 6
print(has_pair_sum([1, 2, 4, 6, 9], 20))  # False

Example

Window example

Problem: Maximum sum of any k consecutive elements. This is a fixed-size sliding window.

First build the first window. Then slide by adding the new item and removing the item that left.

def max_sum_k(nums, k):
    if k <= 0 or k > len(nums):
        return None

    window_sum = sum(nums[:k])
    best = window_sum

    for right in range(k, len(nums)):
        window_sum += nums[right]
        window_sum -= nums[right - k]
        best = max(best, window_sum)

    return best


print(max_sum_k([2, 1, 5, 1, 3, 2], 3))  # 9, from [5, 1, 3]

Tip

Write the explanation

In your README, write why each pointer moves. This is what an interviewer checks when you talk through the solution.

Use AI to draft the explanation if you want. Then check that it states the invariant, the movement rule, and the time complexity correctly.

Problem: Pair sum in sorted array
Pattern: Two pointers
Idea: Keep left at the smallest value and right at the largest value.
Move left when the sum is too small. Move right when the sum is too large.
Why it works: The array is sorted, so these moves are the only ones that can fix the sum.
Time: O(n)
Space: O(1)

Two pointers and windows

One idea

When to use

Pick the pattern

Common mistake

Example solution

Window example

Write the explanation

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