How to Solve Next Greater Element on LeetCode
The Next Greater Element problem tests your understanding of arrays, stacks, and efficient search for "next" relationships. It's common in technical interviews, especially for stack and array mastery.
Problem Statement
Given an array, for each element, find the next greater element to its right. If none, return -1 for that element.
Example:
Input: nums = [2, 1, 2, 4, 3]
Output: [4, 2, 4, -1, -1]
Why This Problem Matters for Interviews
This problem:
- Shows how well you can use stacks to optimize for O(n)
- Tests your logic with array indices and element relationships
- Introduces the monotonic stack concept
Optimal Approach: Monotonic Stack
Using Stack to Track Candidates
Iterate from right to left, maintaining a decreasing stack.
Time Complexity: O(n)
Space Complexity: O(n)
def nextGreaterElements(nums: list[int]) -> list[int]:
res = [-1] * len(nums)
stack = []
for i in range(len(nums)-1, -1, -1):
while stack and stack[-1] <= nums[i]:
stack.pop()
if stack:
res[i] = stack[-1]
stack.append(nums[i])
return res
Key Interview Talking Points
- Brute force (O(n²)) vs. stack-based (O(n)).
- Why the stack works (monotonic decreasing property).
- How this idea extends to circular arrays and other problems.
- Discuss handling duplicates and edge cases.
Big O Complexity Recap
| Approach | Time Complexity | Space Complexity |
|---|---|---|
| Brute Force | O(n²) | O(1) |
| Stack-based | O(n) | O(n) |
Pro Interview Tips
- Trace your code with sample inputs during interviews.
- Mention how to adapt this for "Next Greater Element II" (circular arrays).
- Practice with test cases