Merge Intervals in Java (Medium) 🔥 | Beats 97% on Leetcode | Sorting + Merging Explained!

📝 Problem Overview:
Problem: You’re given an array of intervals [[start1, end1], [start2, end2], ...]. Your goal is to merge overlapping intervals and return a new array of merged intervals.
Key Example:
Input: [[1, 3], [2, 6], [8, 10], [15, 18]]
Output: [[1, 6], [8, 10], [15, 18]]
The intervals [1, 3] and [2, 6] overlap, so they are merged into [1, 6].
Constraints:
Intervals may not be sorted.
No assumptions about overlap behavior can be made beforehand.
💡 Pseudo Logic for the Solution
Here’s the step-by-step approach to solving the problem:

Sort the Intervals:

Why? Sorting intervals by their start time ensures that overlapping intervals will be adjacent, making the merging process straightforward.
Sorting complexity is O(N log N).
Iterate Over Sorted Intervals:

Use a result list to store merged intervals.
For each interval:
If the list is empty or the current interval does not overlap with the last merged interval, add the current interval to the result list.
Otherwise, merge the intervals by updating the end time of the last interval using:
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Convert the Result List to a 2D Array:

After processing all intervals, convert the List int[] into a int[][] format for the final output.

🔍 Detailed Steps (Interview Perspective)
Understand the Problem:

What defines overlap? Two intervals overlap if interval2[0] less than = interval1[1].
What should be returned? Only merged intervals, ensuring no overlaps exist in the output.
Plan the Solution:

Data Structure Choice: Use an ArrayList int[] to store results, as it’s memory efficient and allows fast access.
Edge Cases to Consider:

Empty input array → Return an empty array.
Single interval → Return the interval as-is.
Fully overlapping intervals → All intervals merge into one.
Optimize the Solution:

Minimize unnecessary list operations. Use a single reference (lastInterval) to track and update the last merged interval.
The key operation in merging overlapping intervals is updating the end time. By using:

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we ensure:

The largest end time is used, which correctly covers all overlapping intervals.
Without this, merging may shrink intervals incorrectly, leading to invalid results.
📚 Identifying Variations of This Problem
Insert Interval (Leetcode 57):

Add a new interval to a sorted set of non-overlapping intervals, then merge if necessary.
Approach: Binary search to find the position, then merge.
Non-overlapping Intervals (Leetcode 435):

Remove the minimum number of intervals to ensure no overlaps remain.
Focus on sorting by end time for efficient merging.
Minimum Arrows to Burst Balloons (Leetcode 452):

Similar logic but applied to finding the fewest cuts or overlaps in intervals.
🧠 Key Takeaways for Solving Interval Problems
Always sort intervals by start time.
Check for overlap conditions: current[beginTime] less than= last[endTime].
Identify the problem pattern: merging, inserting, or minimizing overlaps.
📌 One-Stop Shop for All Resources
🎥 Timestamps for Easy Navigation
0:00 - Intro & Problem Explanation
1:30 - Pseudo Logic & Interview Tips
3:50 - Sorting & Overlap Conditions
6:20 - Merging Intervals (Java Code Walkthrough)
9:00 - Edge Cases & Variations of Merge Intervals
12:30 - Key Takeaways & Next Steps

🎯 Target Audience
CS Students learning algorithms and data structures.
Leetcode Beginners mastering interval-related problems.
FAANG Aspirants preparing for system design and algorithm interviews.
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