Two Pointers on interval list
Last updated: August 15, 2025
Quick Overview
Given a list of intervals, use the two-pointer technique to merge overlapping intervals and return a new list of non-overlapping intervals. The input is a list of intervals represented as pairs of integers, and the output should be a list of merged intervals sorted by their start times. Ensure that the solution runs in O(n log n) time complexity due to the sorting step.
Citadel
August 15, 2025154
6
398 solved
Given a list of intervals, use the two-pointer technique to merge overlapping intervals and return a new list of non-overlapping intervals. The input is a list of intervals represented as pairs of integers, and the output should be a list of merged intervals sorted by their start times. Ensure that the solution runs in O(n log n) time complexity due to the sorting step.
Citadel uses this problem in the Technical Screen to evaluate your algorithmic thinking. They expect you to discuss multiple approaches, analyze trade-offs between them, and implement the optimal solution with clean, readable code.
What the Interviewer Expects
- Recognize the underlying problem pattern (sliding window, two pointers, BFS/DFS, etc.)
- Discuss multiple approaches and trade-offs before coding
- Implement an optimal solution with clean, production-quality code
- Handle all edge cases including boundary conditions and invalid input
- Optimize both time and space complexity with clear justification
- Test your solution systematically with well-chosen examples
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- Can you solve this in a single pass?
- Can you solve this iteratively instead of recursively (or vice versa)?
- How would you modify your solution to handle streaming input?
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Practice DSA ProblemsSample Answer
Problem Analysis
In this problem, we are tasked with merging overlapping intervals from a list of intervals. The two-pointer technique is applicable here as it allows us to efficiently traverse the sorted list of inte...
Approach
- Sort the Intervals: Start by sorting the list of intervals based on the first element (start time). For example, if the input is
[[1,3],[2,6],[8,10],[15,18]], after sorting it remains the sam...
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