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Pinduoduo·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

Pinduoduo SWE interview with a classic next greater element problem. Pretty standard algorithmic round, nothing too wild, but it's the kind of question where you either know the pattern or you're fumbling around with nested loops hoping they don't notice.

Questions Asked (1)

Q1

Given two arrays where the first is a subset of the second, find the next greater element for each value from the first array as it appears in the second array. Return -1 if no greater element exists to the right.

Algorithms & Data Structures
Author's notes

The brute force is obvious and they'll let you say it out loud, but you better pivot fast.

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AI HintsAI Generated

Suggested Approach

Use a monotonic decreasing stack to compute the next greater element for all elements in the second array in O(n) time, storing results in a hash map. Then iterate through the first array and look up each element's next greater element from the map, returning -1 if not found.

Pro tip: Clarify that the first array is a subset of the second, so every element in the first array exists in the second. This allows you to precompute for the entire second array and then answer queries in O(1) per element.

1. Understand the problem and constraints

Restate the problem: for each element in nums1, find the first greater element to its right in nums2. Confirm that nums1 is a subset of nums2 and that elements are distinct.

2. Choose the optimal data structures

Select a monotonic stack to efficiently find the next greater element for each element in nums2, and a hash map to store the results for O(1) lookups.

3. Compute next greater elements for nums2

Iterate through nums2 from left to right, maintaining a decreasing stack. For each element, pop smaller elements from the stack and record the current element as their next greater element in the map. Push the current element onto the stack.

4. Build the result for nums1

Iterate through nums1, retrieve the next greater element from the map for each value, and use -1 if the value is not in the map.

5. Analyze complexity and edge cases

State that the time complexity is O(n + m) and space complexity is O(n), where n and m are the lengths of nums2 and nums1. Discuss edge cases like no greater element or empty arrays.

Key Points to Mention

  • Monotonic stack technique for next greater element
  • Hash map for O(1) lookups
  • Time and space complexity analysis
  • Handling elements with no greater element (return -1)
  • Assumption that nums1 is a subset of nums2
  • Iterating from right to left vs left to right (both valid, but left to right with stack is common)

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.