Shortest Unsorted Continuous Subarray (Array/Pointers)
Given an integer array, you need to find one continuous subarray that, if you only sort this subarray, the whole array will be sorted. Find its length.
Why Interviewers Ask This
Google interviewers ask this to assess your ability to optimize beyond brute force. They evaluate if you can identify edge cases where sorting the entire array is inefficient. The problem tests your mastery of two-pointer techniques and your skill in analyzing time complexity, specifically distinguishing between O(n log n) sorting approaches versus optimal O(n) linear scans.
How to Answer This Question
1. Clarify the goal: Confirm that sorting only a specific subarray must result in the entire array being sorted. 2. Analyze constraints: Acknowledge the need for an O(n) solution rather than naive O(n log n) sorting. 3. Identify boundaries: Explain how to find the start index by scanning left-to-right to find the first element out of order relative to the maximum seen so far. 4. Define end points: Describe scanning right-to-left to locate the last element smaller than the minimum seen from the right. 5. Calculate length: Subtract the indices to get the final answer. Throughout, explicitly mention handling edge cases like already sorted arrays or single-element mismatches to demonstrate thoroughness.
Key Points to Cover
- Demonstrating awareness of O(n) vs O(n log n) complexity trade-offs
- Using two-pass logic to define exact boundary indices without sorting
- Handling edge cases like pre-sorted arrays or duplicate values
- Clearly articulating the relationship between local disorder and global order
- Maintaining constant space complexity while achieving linear time
Sample Answer
To solve this efficiently, I would avoid full sorting since it takes O(n log n). Instead, I aim for an O(n) approach using two passes. First, I'll traverse from left to right, tracking the maximum value encountered. If I…
Common Mistakes to Avoid
- Immediately suggesting full array sorting which misses the O(n) requirement
- Failing to handle the case where the array is already perfectly sorted
- Confusing the direction of comparisons when tracking min/max values
- Not accounting for duplicate numbers which can skew boundary detection
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