Add Next Greater Element algorithm

[Nikita20010]

Summary

Added a Python implementation for the Next Greater Element problem using a stack-based approach.

Details

• Implemented next_greater_elements() function in data_structures/stacks/next_greater_element.py
• Time Complexity: O(n)
• Space Complexity: O(n)

Example

Input: [4, 5, 2, 25]
Output: [5, 25, 25, -1]

[algorithms-keeper]

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