The core idea clicked pretty fast for me: scan the second array with a monotonic decreasing stack and build a hash map as you go, then just look up each element from the first array.
Start by clarifying the problem constraints (e.g., array sizes, duplicates) and then propose an efficient solution using a monotonic stack to precompute the next greater element for all elements in the second array. Store these results in a hash map, then iterate through the first array to build the output by looking up each element in the map.
Pro tip: Mention that the monotonic stack approach runs in O(n + m) time and O(m) space, which is optimal, and briefly discuss how you would handle edge cases like duplicates or missing elements to show thoroughness.
Ask about constraints: array sizes, whether elements are unique, and if the first array is a subset with all elements present in the second. Confirm that 'next greater' means the first greater element to the right.
Briefly describe a naive O(n*m) solution: for each element in the first array, scan the second array to find the next greater element. Acknowledge its inefficiency to set up the optimized solution.
Explain how to traverse the second array from right to left, maintaining a decreasing stack. For each element, pop smaller elements, the top of the stack (if any) is the next greater element; store this in a hash map.
Iterate through the first array, and for each element, retrieve its next greater element from the hash map (or -1 if not found). Return the resulting array.
State time complexity O(n + m) and space O(m). Discuss handling duplicates (if present, map to the first occurrence or adjust stack logic) and elements with no greater element.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.