Next Greater Element & Stock Span Problem

Monotonic Stack Pattern

Maintains elements in strictly increasing or decreasing order. Eliminates nested loops, reducing time from $O(N^2)$ to $O(N)$.

Production Code Example:

import java.util.*;
public class NextGreater {
    public int[] nextGreaterElements(int[] nums) {
        int n = nums.length;
        int[] res = new int[n];
        Arrays.fill(res, -1);
        Stack<Integer> st = new Stack<>();
        for (int i = 0; i < n; i++) {
            while (!st.isEmpty() && nums[st.peek()] < nums[i]) {
                res[st.pop()] = nums[i];
            }
            st.push(i);
        }
        return res;
    }
}

Key Complexity & Algorithmic Takeaways:

When implementing Next Greater Element & Stock Span Problem in coding interviews and production applications, keep these core guidelines in mind:

• Time Complexity Analysis: Always evaluate best-case, average-case, and worst-case time complexities ($O(1)$, $O(\log n)$, $O(n)$, $O(n \log n)$, $O(n^2)$).
• Space Complexity & Memory Bounds: Account for auxiliary memory usage, call stack frame recursion overhead, and heap allocations.
• Edge Cases & Validation: Test empty inputs, null pointers, single-element collections, duplicate values, and integer overflow bounds.
• Optimal vs Naive Solutions: Start with a clear brute-force solution, then optimize using techniques like Hashing, Two Pointers, Windowing, or Dynamic Programming.

Summary Takeaway:

Mastering Next Greater Element & Stock Span Problem provides the foundational problem-solving skills needed to pass technical coding interviews at top tech companies and write ultra-performant software systems.