Triplet Sum (3Sum)
Find all unique triplets in an array whose values sum to zero (LeetCode's 3Sum).
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Lesson notes
Try it before you watch
Restate the problem in your own words, list the edge cases, and sketch a solution with its running time. Then play the video and compare.
Reveal the key idea
Sort the array, fix one element, and run a two-pointer search for the remaining pair. Skip equal neighbours to avoid duplicate triplets.
Pattern: Two pointers & sliding window. Move two indices through a sequence so each element is visited a constant number of times.
Complexity
| Measure | Bound |
|---|---|
| Time | O(n²) |
| Extra space | O(1) extra, excluding sorting and output |
Walkthroughs often start from a simpler approach first; aim to reach these bounds. New to Big-O? Read understanding algorithmic complexity.