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1def two_sum(nums, target):

2 seen = {}

3 for i, num in enumerate(nums):

4 comp = target - num

5 if comp in seen:

6 return [seen[comp], i]

7 seen[num] = i

8 return []

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Array

Auto Visualizations

Your data structures draw themselves

Arrays, hash maps, linked lists, trees, graphs — they all render automatically as your code executes. Pointers sweep. Nodes light up. Edges form. No config needed.

Guided Lessons

The problems interviewers actually ask

Step-by-step walkthroughs of real interview algorithms. Key lines are highlighted and tricky steps are annotated.

Guided Lessons

Two SumArrays

9 steps

Completed

Valid ParenthesesStacks

12 steps

60%

Merge SortSorting

15 steps

BFS TraversalGraphs

11 steps

Room

abc123

1def search(arr, target):

Alice

2 for i in range(len(arr)):

3 if arr[i] == target:

Bob

4 return i

5 return -1

tracecode.dev/room/abc123

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Watching them run is.

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Finally understand the algorithm.

Your data structures draw themselves

The problems interviewers actually ask

Practice togetherSoon

Reading solutions isn't learning. Watching them run is.

Join the beta

Finally understand the algorithm.

Your data structures draw themselves

The problems interviewers actually ask

Practice togetherSoon

Reading solutions isn't learning. Watching them run is.

Join the beta

Finally understand
the algorithm.

Reading solutions isn't learning.
Watching them run is.

Finally understand
the algorithm.

Reading solutions isn't learning.
Watching them run is.