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ALGOCADEMY DATA · NOVEMBER 2026

Which coding interview problems do learners fail most?

Every time a learner pressed Check on AlgoCademy, March 2020 to November 2026. Free to use with a link.

Code checks
1,164,307
Learners
38,094
Checks that fail
43%
Pass Two Sum on the first try
26%

Alone, 23% pass an interview problem on the first try

A learner’s first full solution, written without our step-by-step guide. 73% get there in the end.

First tries at interview problems, hardest first Passed on the first tryPassed on a later tryNever passed
  1. Number of IslandsGraphs · 114 learners4%25%
  2. Longest Consecutive SequenceSorting · 67 learners4%66%
  3. Maximum Sum SubarrayPrefix sums · 343 learners11%61%
  4. Find DuplicatesHash tables · 90 learners11%87%
  5. Longest Subarray Without Repeating ValuesBrute force · 98 learners11%65%
  6. Pair CountBrute force · 81 learners14%85%
  7. Post Order Tree TraversalBinary trees · 70 learners16%73%
  8. Palindrome CheckStrings · 80 learners20%59%
  9. ZigZag Tree TraversalBinary trees · 120 learners23%74%
  10. Reverse StringStrings · 90 learners24%86%
  11. Two SumBrute force · 512 learners26%75%
  12. Distinct ValuesSorting · 80 learners28%88%
  13. Remove Duplicates from ArrayBrute force · 178 learners30%81%
  14. Smallest KSorting · 347 learners38%77%
  15. Binary Strings Without Consecutive OnesDynamic programming · 62 learners56%89%
  16. Binary Strings of Given LengthDynamic programming · 126 learners69%92%

Problems with at least 60 learners. A check passes when the code passes every test. The margin of error is 3 to 12 points per row.

Graphs and binary search fail most

The same first tries, grouped by topic.

Share of first tries that pass, by topic
  1. Graphs175 first tries6%
  2. Binary search72 first tries7%
  3. Intervals70 first tries9%
  4. Two pointers95 first tries9%
  5. Prefix sums514 first tries11%
  6. Hash tables129 first tries12%
  7. Linked lists68 first tries16%
  8. Binary trees310 first tries19%
  9. Backtracking77 first tries22%
  10. Strings217 first tries22%
  11. Brute force926 first tries24%
  12. Sorting520 first tries31%
  13. Dynamic programming425 first tries48%

Topics with at least 60 first tries. A topic’s number depends on the problems we teach in it: our dynamic programming chapter opens with two gentle counting problems.

In small steps, 70% of first tries pass

The same interview problems, built a few lines at a time with our step-by-step guide.

Whole solution, alone
23%
of 3,763 first tries pass
One guided step at a time
70%
of 137,846 first tries pass

94% try again after a failed first try

65% of them reach a passing solution. Of everyone who passed, this is the try that did it:

Which try passed
  1. 1st try31%
  2. 2nd17%
  3. 3rd11%
  4. 4th8%
  5. 5th6%
  6. 6th4%
  7. 7th4%
  8. 8th3%
  9. 9th2%
  10. 10th or later13%

2,758 solved problems, interview topics, without the guide.

In Java, 64% of failed checks never run

All 501,504 failed checks, beginner lessons included, by what went wrong.

Why checks fail, by language Wrong output or too slowCrashed while runningDidn’t compile
  1. Python234,889 failed checks45% wrong output or too slow, 55% crashed while running
  2. JavaScript89,472 failed checks53% wrong output or too slow, 47% crashed while running
  3. Java105,114 failed checks33% wrong output or too slow, 3% crashed while running, 64% didn’t compile
  4. C++72,029 failed checks45% wrong output or too slow, 2% crashed while running, 53% didn’t compile

Python and JavaScript have no compile step, so their typos count as crashes.

The hardest beginner lesson: joining two strings

28% of learners fail their very first check. These Python lessons have the lowest first-try pass rates.

Share of first tries that pass, Python beginner lessons
  1. String Concatenation3,446 learners21%
  2. Price After Discount1,908 learners26%
  3. Find Discount639 learners35%
  4. Variable Reassignment3,355 learners44%
  5. Intro to Strings10,243 learners47%
  6. If Statements1,956 learners49%
  7. Printing to the Console12,402 learners56%
  8. Maximum Value and Number of Occurrences1,155 learners56%

Lessons with at least 500 learners.

Method and limits

What we counted

  • 1,164,307 checks by 38,094 learners on 430 lessons and problems, in Python, Java, JavaScript and C++, from March 2020 to November 2026.
  • A check runs the learner’s code against our tests. It passes only when every test passes.
  • “Alone” means without the step-by-step guide, by learners who hadn’t used the guide on that problem before. Untouched starter code isn’t counted.
  • “First try” is a learner’s first such check on a problem. “In the end” is any later passing check.
  • Margins of error are 95% Wilson intervals. They are in the files below.

What it can’t tell you

  • These are people learning on AlgoCademy, many of them beginners. They aren’t a sample of working engineers.
  • Learners choose which problems to try, so each row has a different group behind it.
  • A learner may have read a hint or seen the problem elsewhere before a first try.
  • Our tests include large inputs. The grader reports a wrong result and a too-slow one as the same failure.
  • Problem names are ours. Maximum Sum Subarray is the classic maximum subarray problem.

Use this data

Free to quote, chart and republish with a link to this page (CC BY 4.0). Counts only: no learner’s code or identity is in the files.

Files (CSV)

Cite as

AlgoCademy (2026). Which coding interview problems do learners fail most? Data from 1,164,307 code checks. https://algocademy.com/research/

Need another cut of the data? team@algocademy.com

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