Mid Detailed answer Two Pointers DSA & Coding Interviews

How do you solve 3Sum?

Q: How do you solve 3Sum?

Short answer: Sort the array. Fix one number, then use two pointers on the remainder to find pairs that sum to −fixed. Skip duplicates carefully to return unique triplets. Overall O(n²). Interview approach Sort ascending. For i from 0..n-3, skip duplicate nums[i]. Left = i+1, right = n-1; move based on sum vs 0. When sum == 0, record triplet and skip duplicate left/right. Complexity Time O(n²), Space O(1) extra besides output (sorting may use O(log n)). Edge cases to mention Fewer than 3 elements All zeros Many duplicates Common follow-ups 4Sum 3Sum Closest Mistakes to avoid Forgetting to skip duplicates → wrong unique set Using the same index twice Sorting + two pointers is the pattern interviewers expect after Two Sum.

Short answer: Sort the array. Fix one number, then use two pointers on the remainder to find pairs that sum to −fixed. Skip duplicates carefully to return unique triplets. Overall O(n²).

Interview approach

Sort ascending.
For i from 0..n-3, skip duplicate nums[i].
Left = i+1, right = n-1; move based on sum vs 0.
When sum == 0, record triplet and skip duplicate left/right.

Complexity

Time O(n²), Space O(1) extra besides output (sorting may use O(log n)).

Edge cases to mention

Fewer than 3 elements
All zeros
Many duplicates

Common follow-ups

4Sum
3Sum Closest

Mistakes to avoid

Forgetting to skip duplicates → wrong unique set
Using the same index twice

Sorting + two pointers is the pattern interviewers expect after Two Sum.

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