Array-Based Coding Problems

You have learned a lot about arrays: creating them, looping through them, and using methods like map, filter, reduce, and sort. Now it's practice time! 💪

Learning to code is like learning to ride a bicycle 🚲. Reading about it helps a little, but you only get better by actually riding. The same is true for coding problems.

How to Use This Lesson

For every problem:

1. 📖 Read the problem and the example carefully.
2. 🤔 Think about how you would solve it on paper first.
3. ⌨️ Try writing your own code in the playground.
4. ✅ Then check the solution and the explanation.

Don't worry if you can't solve a problem on the first try. Even experienced developers get stuck. What matters is that you try first. 🙌

A Simple Plan for Solving Any Problem

🟢 Level 1: Easy

Problem 1: Sum and Average

Task: Find the total and the average marks of a student.

Input: [78, 85, 92, 66, 74]Output: Sum: 395, Average: 79

💡 Idea: Add all the numbers, then divide the sum by how many numbers there are.

Problem 2: Largest and Smallest Number

Task: Find the highest and the lowest score in a cricket series.

Input: [45, 110, 82, 23, 67]Output: Max: 110, Min: 23

💡 Idea: Start by assuming the first number is both the biggest and the smallest. Then check every number and update if you find a bigger or smaller one.

Problem 3: Count Even and Odd Numbers

Input: [1, 2, 3, 4, 5, 6, 7]Output: Even: 3, Odd: 4

💡 Idea: A number is even if dividing it by 2 leaves no remainder (n % 2 === 0).

Problem 4: Reverse an Array (Without reverse())

Input: [1, 2, 3, 4, 5]Output: [5, 4, 3, 2, 1]

💡 Idea: Swap the first and last items, then move inward from both sides. It's like two people standing at the two ends of a line and walking towards each other, swapping places each step. This is called the two-pointer technique.

[1, 2, 3, 4, 5]
 ↑           ↑    swap 1 and 5 → [5, 2, 3, 4, 1]
    ↑     ↑       swap 2 and 4 → [5, 4, 3, 2, 1]
       ↑          they meet → stop ✅

Problem 5: Check if an Item Exists

Task: Check if a city is on the delivery list.

Input: ["Delhi", "Pune", "Jaipur"], search: "Pune"Output: Pune is available ✅

💡 Idea: Go through each item. If it matches, stop and say "found."

🟡 Level 2: Medium

Problem 6: Remove Duplicates

Task: Remove repeated drinks from an order list.

Input: ["Tea", "Coffee", "Tea", "Juice", "Coffee"]Output: ["Tea", "Coffee", "Juice"]

💡 Idea: Keep an item only the first time you see it.

A Set is a special collection that stores only unique values. Converting an array to a Set automatically removes duplicates.

Problem 7: Count How Often Each Item Appears

Task: Count the votes for each snack at a class party. 🍿

Input: ["Samosa", "Pizza", "Samosa", "Momos", "Pizza", "Samosa"]Output: { Samosa: 3, Pizza: 2, Momos: 1 }

💡 Idea: Use an object like a tally sheet. Every time you see a snack, add 1 to its count.

How (count[snack] || 0) + 1 works:

• First time we see "Samosa" → count["Samosa"] is undefined → undefined || 0 gives 0 → 0 + 1 = 1
• Next time → count["Samosa"] is 1 → 1 + 1 = 2

Problem 8: Find the Second Largest Number

Input: [10, 45, 32, 45, 28]Output: 32

💡 Idea: Keep track of the top two numbers while going through the list once. Be careful: 45 appears twice, but the second largest is still 32.

Another way (simpler, but slower for big arrays):

Problem 9: Find the Missing Number

Task: Roll numbers from 1 to 6 should be present, but one student is absent. Find the missing roll number.

Input: [1, 2, 4, 5, 6]Output: 3

💡 Idea: The sum of numbers from 1 to n is n × (n + 1) ÷ 2. Find what the sum should be, subtract what it actually is, and the difference is the missing number! 🧠

Problem 10: Move All Zeros to the End

Input: [0, 1, 0, 3, 12]Output: [1, 3, 12, 0, 0]

💡 Idea: Separate the non-zero numbers and the zeros, then put them back together. The order of the non-zero numbers must stay the same.

Problem 11: Find Common Items in Two Arrays

Task: Find the friends who are coming to both parties. 🎉

Input: ["Amit", "Neha", "Ravi", "Sara"] and ["Ravi", "Kiran", "Amit"]Output: ["Amit", "Ravi"]

💡 Idea: Keep only the items from the first list that are also in the second list.

Problem 12: Split an Array into Groups (Chunks)

Task: Divide students into teams of 2 for a project.

Input: ["A", "B", "C", "D", "E"], team size: 2Output: [["A", "B"], ["C", "D"], ["E"]]

💡 Idea: Jump forward by the team size each time, and slice() out one group.

🔴 Level 3: Interview Favourites

Problem 13: Two Sum ⭐

Task: Find the positions of two numbers that add up to a target.

Input: nums = [2, 7, 11, 15], target = 9Output: [0, 1] (because 2 + 7 = 9)

This is one of the most famous coding interview questions!

Approach 1: Check every pair (simple, but slow)

💡 Idea: Try every possible pair of numbers.

This works, but for 1,000 numbers it checks about 500,000 pairs! 🐢

Approach 2: Remember what you've seen (fast)

💡 Idea: For each number, ask: "Have I already seen the number I need to reach the target?" Keep a notebook (an object) of numbers you've seen, and where you saw them.

target = 9
see 2 → need 7 → not seen yet → note down: { 2: 0 }
see 7 → need 2 → seen at index 0! ✅ → answer [0, 1]

This checks each number only once. 🚀

Problem 14: Rotate an Array

Task: Rotate the array to the right by k steps.

Input: [1, 2, 3, 4, 5], k = 2Output: [4, 5, 1, 2, 3]

💡 Idea: Take the last k items and move them to the front. Like a merry-go-round 🎠: each step, the last seat comes around to the front.

Problem 15: Merge Two Sorted Arrays

Input: [1, 4, 7] and [2, 3, 8]Output: [1, 2, 3, 4, 7, 8]

💡 Idea: Imagine two queues, both sorted by height. Compare the front person in each queue, and send the shorter one forward first. Repeat until both queues are empty.

Problem 16: Maximum Profit from Stock Prices 📈

Task: You are given the price of a stock for each day. Buy on one day and sell on a later day. Find the maximum profit.

Input: [7, 1, 5, 3, 6, 4]Output: 5 (buy at 1, sell at 6)

💡 Idea: Go day by day. Remember the lowest price seen so far. On each day, check: "If I sell today, how much profit would I make?"

Problem 17: Maximum Sum of a Continuous Part (Kadane's Algorithm)

Task: Find the largest sum of any continuous part of the array.

Input: [-2, 1, -3, 4, -1, 2, 1, -5, 4]Output: 6 (from the part [4, -1, 2, 1])

💡 Idea: Walk through the array, keeping a running total. If the running total becomes negative, it will only pull the next numbers down, so start fresh from the next number.

It's like a road trip 🚗: if you've lost more than you've gained so far, forget the past and restart the journey from here.

🎯 Practice Challenges (Try Yourself!)

No solutions here. Use the playground and try them on your own! 💪

1. Is it sorted? Check if an array is in ascending order. (Hint: every())
2. Product of all numbers: Multiply all the numbers in an array. (Hint: reduce() starting with 1)
3. Longest word: Find the longest word in ["Delhi", "Thiruvananthapuram", "Goa"].
4. Group by first letter: ["apple", "avocado", "banana", "blueberry"] → { a: [...], b: [...] }
5. Running total: [1, 2, 3, 4] → [1, 3, 6, 10]
6. Find all pairs that add up to 10 in [1, 9, 3, 7, 5, 5].
7. Leaders: Find all numbers that are bigger than every number to their right in [16, 17, 4, 3, 5, 2] → [17, 5, 2].

Common Patterns You Just Learned

Tips for Solving Array Problems

• ✅ Always test with edge cases: an empty array [], one item [5], negative numbers, and duplicates.
• ✅ Write a simple solution first, even if it's slow. Make it faster later.
• ✅ Use clear variable names like highest, total, and seen, not x or temp.
• ✅ Use console.log() inside loops to see what is happening at each step.
• ✅ Practice a little every day. Consistency beats a long session once a week. 🚀

Key Takeaways

• Solving problems is the best way to master arrays. Try first, then check the solution.
• Most array problems use a few common patterns: running totals, tracking the best value, tally objects, two pointers, and Sets.
• reduce() is great for totals and counts, while filter(), includes(), and Set make many problems short and simple.
• A slow solution that works is a great start. Then look for a faster idea, like in Two Sum.
• Break every problem into small steps, and solve them one at a time. 🧩