Problem database last updated: June 20, 2025
4 problems · 0 Easy, 3 Medium, 1 Hard · Ranked #273 of 458
0 Easy
0% · avg 23%
3 Medium
75% · avg 59%
1 Hard
25% · avg 18%
Based on 4 reported problems, Apollo.io interviews are slightly harder than average - 25% Hard vs 18% across all companies. The majority (75%) of questions are Medium difficulty, which is typical for companies that want to see solid fundamentals without excessive trick questions.
Compared to the industry average, Apollo.io puts unusual emphasis on stack (50% of problems, 5.8x the industry average), hash-table (50% of problems, 2.2x the industry average). If you're short on time, these are the categories to double down on.
The most common topics are array (75%), hash-table (50%), stack (50%), string (25%). Problems below are sorted by frequency, the ones at the top are asked most often.
| Problem | Difficulty | Frequency | Topics | |
|---|---|---|---|---|
Minimum Window Substring Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is inc... | Hard | Very Likely | hash-tablestringsliding-window | Solve |
Validate Stack Sequences Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on... | Medium | Very Likely | arraystacksimulation | Solve |
Subarray Sum Equals K Given an array of integers nums and an integer k, return the total number of subarrays whose sum equals to k. | Medium | Very Likely | arrayhash-tableprefix-sum | Solve |
Evaluate Reverse Polish Notation You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation. | Medium | Very Likely | arraymathstack | Solve |
Minimum Window Substring
SolveGiven two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is inc...
Validate Stack Sequences
SolveGiven two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on...
Subarray Sum Equals K
SolveGiven an array of integers nums and an integer k, return the total number of subarrays whose sum equals to k.
Evaluate Reverse Polish Notation
SolveYou are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.
Frequency scores are based on crowdsourced interview reports. A higher score means the problem has been reported more often in recent Apollo.io interviews.
Very Likely
75-100%
Likely
50-74%
Sometimes
25-49%
Rare
0-24%
Apollo.io interviews focus heavily on array, hash-table, stack problems. If you're short on time, these are the categories to prioritize. The problems on this page are sorted by frequency, so start from the top and work your way down.
Beyond solving problems, practice explaining your approach. Apollo.io interviewers care about your thought process - how you break down a problem, consider edge cases, and evaluate tradeoffs between solutions. A clean O(n) solution you can explain clearly beats an O(log n) solution you can't articulate.
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Apollo.io has been reported to ask 4 distinct coding problems. The most common topics are array, hash-table, stack. 0 are Easy difficulty, 3 are Medium, and 1 are Hard. Problems are sorted by frequency - the ones at the top are asked most often.
Based on 4 reported problems, Apollo.io interviews are slightly harder than average - 25% Hard vs 18% across all companies. 75% of questions are Medium difficulty. Focus on the high-frequency Medium problems first, then work through the Hard ones.
Start with the highest-frequency problems listed on this page. Focus on the core topics: array, hash-table, stack. Practice solving them under time pressure and explaining your approach out loud. Mock interviews with AI can simulate the real experience.
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