How do you find the next greater element for each item in an array?
A fundamental problem requiring you to find the first element to the right that is larger than the current element for every item.
Why Interviewers Ask This
This is a standard interview question to test proficiency with stacks and the concept of finding the next greater element, a pattern recurring in many optimization problems.
How to Answer This Question
Explain the brute force nested loop approach. Introduce the optimal solution using a stack to store indices of elements waiting for a greater value. Iterate through the array; for each element, pop from the stack while the current element is greater. Assign the current element as the answer for popped items. Push the current index onto the stack. Conclude with O(N) time complexity.
Key Points to Cover
- Utilize a stack to store indices
- Pop elements when a greater value is found
- Handle elements with no greater successor
- Optimize from O(N^2) to O(N)
Sample Answer
I would solve this using a monotonic stack to store indices of elements for which we haven't found a greater element yet. As I iterate through the array, I compare the current element with the element at the index on top…
Common Mistakes to Avoid
- Forgetting to initialize the answer array with -1
- Popping elements incorrectly
- Not handling the remaining elements in the stack
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