How do you find the next greater element for each element in an array?
This problem requires finding the first larger element to the right for every item. It is a standard application of the monotonic stack technique.
Why Interviewers Ask This
Finding the next greater element is a fundamental pattern used in various real-world scenarios like stock analysis and resource allocation. It tests the ability to apply stack-based solutions for range queries.
How to Answer This Question
Iterate through the array and maintain a stack of indices for which the next greater element hasn't been found yet. For each new element, check if it is greater than the element at the stack's top index. If so, record the answer and pop. Push the current index onto the stack.
Key Points to Cover
- Maintain stack of indices
- Check current against stack top
- Pop when greater element found
- O(N) time complexity
Sample Answer
I will use a stack to keep track of indices. As I iterate through the array, if the current element is greater than the element at the index stored at the top of the stack, I have found the next greater element. I pop the index and record the current element as the answer. I repeat this until the stack is empty or the condition fails, then push the current index.
Common Mistakes to Avoid
- Not initializing result array
- Processing elements in wrong order
- Forgetting to fill -1 for no greater element
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